Yes. And it is also possible do on Enterprise.
If my theory right about the EC lines in Hires 2 -> 16 color then possible 640x resolution with 16 colors on external graphics! (736x300 if full borderless)
But for this need some hw acceleration: scroll registers, block operations (copy, fill, etc)...
Plus the sprites
This was my idea too, SymbOS would look great for example
Basically it's almost the user experience of GFX9000 etc then
Though those stuffs are fast enough even using "private to the video chip" VRAM, since there is some hardware acceleration, what you say above exactly, Zozo. It would be rather interesting (though a complex project, I guess it would need an FPGA already ...) to implement something like the video chip used by MSX2 or even better the V9990 (or what is that which is used in GFX9000?). The extra value of this, if it's implemented in a way, that the "main screen" of the emulated entity can be even turned off, and just its sprites remains:
* there can be a Nick + sprites like configuration, by switching the "main external screen" off, but not the sprites
* maybe even MSX emulation (!) on EP at some level at least, since then the external gfx unit can be MSX/MSX2 compatible, or whatever
* you can even mix Nick/external gfx if the latter can use the /EXTC line from memory too
* other tricky mixes, by using the two bits on port 0x80 which controls the process
* to simply get a nice, high resolution screen as whole, ie that 736x300 16 colour and similar
Then it's kinda multi-purpose stuff, however this is far more than a "little logic" ... Unfortunately - I think - a real V9958 or such cannot be used, as it hasn't got digital (?) outputs to feed on external colour inputs of EP, and even if it has, maybe it's not possible to "sync" on the video clock (and custom vsync, dictated by vsync mode LPBs etc) of Nick ...
Surely, there are limits, like the 16 colour, a V9958 and V9990 would know more than "only" 16 colours, but that's another question.