i was going to ask piru and karlos if there was a way to use the mmu to remap code that wants to use chip to fast but you would probably jump all over that too.
This shouldn't really be needed, certainly not for an application like Dpaint IV - after all it is 'new' in comparison to programs that actually don't already use fast ram for everything that it can.
If you look at a basic OS1.3 (maybe it is since 2.0) AmigaOS install and above, you can find a system tool called "NoFastMem". This was to assist very old programs (much older than DP IV) that for various reasons don't work if their code is loaded into fast ram. You ran NoFastMem, which temporarily 'hid' the fast memory, then you ran your troublesome application which was then forced to use only chip ram for its code and data. Then you could safely run nofastmem again to 'unhide' the fast memory.
DPaint uses lots of chip ram because of the type of application it is - an art package designed for native video modes. It uses chip memory for the screen, some hidden images, brushes, stencils, animation frames and lots of other stuff. These things are either displayed or operated on using the blitter, which is why they need to be in chip ram.
However, it really ought not to be using any chip ram for anything that can be safely put into fast. By which, of course, I mean anything the custom chips don't need access to, such as program code and data structures - even graphics data that is not presently 'in use' can be stored in fast ram if the program knows how to page it in and out.
What you really need is a paint package that does everything using the CPU and does it in fast ram. Only the display itself needs to be in Chip ram (or indeed video memory on a graphics card) and you would have to have a code that performs copies of the changed areas of the image from fast ram to the display area. This would remove your limit but has one overriding drawback - you likely need a much faster CPU to do everything with, more powerful than the average user would have had when the software was originally made.
The other alternative is to have a paint program that only uses graphics.library routines. This would mean FBlit could help a lot on basic AGA and also you could run it under RTG. The drawback here is that the graphics.library routines are frankly rather crap for making a decent paint package with.