as far as networking for hdl goes - i think it might be very difficult if not impossible, because you would need networking driver, and some protocol implementation running in the background (and eventual tcp/ip stack, if necessary).
it still surprises me that most ps2 games do not run out of iop memory under hdloader (or hdl+igr). then again, i'm no ps2 guru ;-)
in case of running ps2 games via network - the simpler, the better, as far as minimizing memory usage goes.
well, as far as ps1 emulator goes - it might be possible, because you have more freedom around IOP, basing on assumption that it uses main ps2 ram for emulation. there is a lower bandwidth requirement, and more resources (ram) for implementing various things.I have however used the beta version of the PS1 emulator PS2PSXe to run PS1 ISOs over LAN using HOST protocol.