There is no software that will hard reset the 790 that I know of. All the MIPS .vb programs that were written don't work with the 790 because some of the hardware registers were not included in the Mobilepro's kernal, ie Tweaks2K, PocketOff, PowerDown, PowerOff Pro, etc...
If the switch is actually absent, then it probably broke off and is floating around in your case somewhere, or someone removed it. If you look at the motherboard where the switch would normally reside, there is an area labeled FRSTSW with 4 contact points. Points 1 & 2 don't do anything, they are just for mounting the switch. Contact 3 and 4 are "normally open." You can solder wires to point 3 and to point 4 and then just let them extend outside the unit somewhere or mount another switch, or actually you can even hardwire them to the keyboard
(I did that on my test unit to the tilde key
) and by grounding those two contacts, you will hard reset your 790...even with the battery intact, even with it plugged in, no matter what. Something you don't want to do inadvertently.
Another option that is easier to get at is to put a simple switch in series with one of the wires to the backup battery that is "normally closed" and then just remove the main battery and open the circuit...that will essentially remove power from the motherboard causing a hard reset as well...and that is actually easier to get to since the wire is easily exposed and doesn't require the precision of soldering as the motherboard hack does.
Or if you don't mind waiting...just remove the main battery!
I understand that the designers meant for a hard reset to be hard to get to, but that has always been one thing I hated on my Mobilepros versus the 720...I liked a simple soft reset hole right on top. I hated having to pull the batteries on the 900 series to hard reset...hence I loved it when Cmonex came up with her software hacks to do the same things....still her hard reset program is useless when the silly thing locks up because I tried to run some WM6 program on it or something. Wish NEC had actually installed some keys that would perform a hard/soft reset by holding down something and hitting another key or something easier....