Never tried, don't know if it's straight forward or tricky as hell (since the PGD-pin is bi-directional for example) but if I were to try it I'd probably try with one of ADUM series digital isolators or something like that.

But what's the application? Why is there a permanently connected PC to the programming port of the PIC?
Is it perhaps possible to initially program the PIC with a bootloader and then, instead of trying to isolate the ICSP pins, use an isolated RS232 connection to the PC? That way you have isolated serial AND programming at the same time.

/Henrik.