I’ve spent a small amount of time toying with this idea in my head, and probably for the better, installing ps2toolkit on my machine keeps bailing.
Here are my thoughts however, informed by the psp port.
The main SH4 cpu shoul be no problem for the PS2’s R5900, assuming it does nothing else. As R5900 has a TLB this would solve most memory related mapping / overhead with similar backpatching approaches as with nullDC on PC.
VU0 should be capable of handling SH4’s fpu unit, as it is far more capable.
TA could be handled with minimal processing in the main core (ala-reicast) and then fully processed in VU1.
Texture caching would be an issue, and it would depend on whenever rendering is done at full resolution or not. VQs could be compressed similar to PSP to paletted textures to save space.
IOP could be used for arm7di and SGC emulation, though it’d barely be up for the task.