There's no reason not to use those pins for both ICSP and normal functions.
You just need to ensure that the whatever they connect to won't take the ICSP signals out of spec' for programming. From what you describe I would think your application will be okay.
Even easier than that - and with no restrictions - you need a socket to plug your ICSP lead in to, so simply give that socket two rows, and plug a dummy plug in that joins the pins across to the required I/O circuit. To program, unplug the link plug (disconnecting the pins entirely), and plug the programmer in - no restrictions, dead easy!.
Should work fine as long as the ICSP also uses pull down resistors (The ICD2 does) and that the loads on the MOSFETs don't mind being toggled during the ICSP process.