Because of how the dialogue system works with SI, it's likely something that could be fixed by the companion mod itself. If the rumors topic is how you handle them, then there is going to be a quest attached to the companion mod that governs what shows up there. It probably has an SEWorld check on it like most other quests to keep Cyrodiil material from showing up in SI. Removing that check on the quest may solve the problem.
This isn't something the USIP can address in any case, and that script mod for companions coming through the gateway won't either.
Yes, according to the Companion Share and Recruit readme, only NPCs with the "Rumors" dialogue option are recruitable. As far as CSR getting another update, I doubt it will happen. TalkieToaster is pretty much done modding for Oblivion for now and has moved on to Fallout 3, AFAIK.
Funny things is, if I recruit anyone from the SI and bring them through to Cyrodiil, they keep their "Rumors" dialogue option after I dismiss them. If I bring someone from Cyrodiil into SI and I dismiss them, they lose their "Rumors" dialouge option. I guess a solution for me, would be to make my own companion NPC and situate them in the SI to start with.