Can you put a toilet against an internal wall?

Yes it's possible.......Think 'macerator' waste pumping system where there is no direct access to the existing toilet waste or the fall would be inadequate as may be in your case, to have 4.5" pipe work installed. Bathroom/plubming stores will be able to put you on right track.
Do a sketch of the area with rough measurements and position of existing 4.5" TOILET waste pipe on the outside wall so they know roughly how far the macerator will have to pump the waste.

NOBBY
April 2009