Turns out yes, you don't want to try this, especially if there is javascript mixed between the two.
a) session merging:
BlueImp uses a flat file for user access. This was quickly traced and re-routed to the correct user table in the login function.
b) output control
since blueImp uses XML for output, using a jQuery div controller simply doesn't work. So I was forced to use the dreaded IFRAME. This works.. up to a point. Any parameters must be passed with GET variables.
c) IE, the dreaded DOM parse.
So sorry, but a single javascript error crashes any and all further code execution. In other words, IFRAMEs can fail if the parent page spawns an error. Suppressing error reporting only works so far.
RESULT it is possible to mix MVCs, but only if you can properly balance all three of the above. Unfortunately, with this project, too many javascript errors blocks the proper functioning with a brick wall. Also, unfortunately, there is no other solution, since chat apps seriously need proper AJAX handshaking to work properly. And well if the parent MVC is crashy, it casades downwards through everything else.
My conclusion, MVCs are nice and all, but PHP function libraries are much nicer... (and also how 99% of the PHP language set works.)
Yes, not a complete solution, but hopefully this will help anyone else that attempts to do this.