As a work-around, you could derive a user control from the Kentico InlineUserControl, and add it to the stylesheet the repeater is using by putting something like %%control.ButtonWrapper.ascx%%, but you'd be limited to passing one parameter.

I have some code that provides better control insertion when parsing XML, it wouldn't take long to replace the kentico xsl processor with it. My version allows either user or server controls, requires no base class, and unlimited number of named parameters. I sent it to Petr, so maybe it'll make it's way in eventually, but I can email you a copy if you'd like.