I haven't heard of anyone getting banned from DAoC just for running it through Inner Space (or WinEQ 2 for that matter) and I wouldn't really expect that to change given that there are no DAoC hacks for Inner Space. I'm not aware of any hostility from Mythic specifically directed at Inner Space. However, if they wanted to detect Inner Space and ban you if they detect it, they could certainly do so and there's nothing preventing them from doing that. Inner Space 1.10 and later up until 1.11 build 5325 wouldn't run DAoC, so only those who would have known to try 1.09 build 5039 could have been using it with Inner Space since then anyway.
They do also have, as the link quoted, a catch-all rule that says they can ban you any time for any reason in their sole discretion:
16. Engage in any behavior that is contrary to the 'spirit of the game' as defined by Mythic in its sole discretion.
As far as how they might detect keystroke broadcasts, who knows. Presumably, this would have to be a server-side mechanic or it would be pretty much useless (meaning that you would be able to work around it by using multiple computers, or virtual machines, etc if it is not server-side). I would therefore have to assume they'd be comparing timestamps of (and/or in) messages sent to the server, e.g. spell casts -- likely against some subset of players, such as your party or characters within some distance from you. And then they perform an investigation that doesn't involve timestamps -- possibly directly observing you in the game. They would have to rule out having multiple keyboards and using two hands to push a button at the same time on each (which is supposed to be perfectly acceptable).
Hardware is always a huge loophole, and one of the main reasons why there should not be a distinction between software multiboxing and hardware (noting that DAoC rules say both software and hardware, when used to broadcast to more than one window per keystroke, are prohibited). EVE used to allow hardware but not software until this story went around the world a few times:
http://kotaku.com/5514947/online-gaming-rig-redefines-the-meaning-of-multiplayer. This guy was/is an ISBoxer user. Anyway, if they were to specifically detect any particular software on the client side, this would give an implicit advantage to anyone who has extra PCs, keyboards and mice to throw at the problem. You just can't "detect" that rig, and even if they tried, it wouldn't be anything a few more dollars couldn't take care of.
Of course, you've got 10 fingers and you could also throw in some foot pedals, and if you want to hold them at their word that, according to that article, you should be allowed to send a keystroke to 5 different windows if you're whacking 5 keys instead of one. Aaaand then all it takes is a popsicle stick (or dowel, as pictured in that link above) to link them together and now you've skirted the "one keystroke" policy. Then our question is, why is it against the rules to build the popsicle stick into the keyboard's software, but not to put the popsicle stick on top of the hardware? Why does it matter how many keys you're whacking in the first place?
The long and the short of it is this: It's hard to detect on any reasonable level, so they would rely mostly on either detecting specific software (dumb, and unless you find reports of this happening, probably isn't happening) or player reports. In a non-PVP setting, they would be biting the hands that feeds them if they were to proactively detect and ban people for multboxing, and to top that there would be mistakes that they would have to apologize for, costing themselves additional time and money. Example: WoW accidentally, automatically banned people for using WinEQ 2 once (intending to ban for something else entirely), and Blizzard had to revert those bans and the victims free subscription time. Hell, Aion banned a significant portion of their userbase for nothing at all a couple weeks after its launch. So while these companies loooooove their detection schemes, detections don't come problem-free.
Anyway, your primary concern as a multiboxer in DAoC should probably be being seen. By anyone at all. If you stay out of anyone else's way, there's not actually a good reason for someone to report you (other than the sheer joy they would experience if they were to get you banned) and it would be utterly stupid, on a business level, to ban your accounts. If you plan to PVP as a multiboxer in DAoC, even if you're not broadcasting keystrokes, you're at risk of "investigation" -- but I'm certain most DAoC players would realize that... considering how much of the game's population considers even having a second account to be cheating.
If it were me, I would probably just move to a different game, like the EQ2 free-to-play (aka F2P), LOTRO and Dungeons & Dragons are F2P and so on. Hell, EQ2 also has this broadcasting rule but they're not going to invoke it unless you're ruining someone else's day and they've even pretty much said that (
http://eq2wire.com/2009/12/22/son-of-banhammer/). Most games don't specifically disallow multiboxing or broadcasting, but will instead invoke their rules on griefing/ruining someone's day -- EA/Mythic would probably do themselves a favor to follow that lead.