One unfortunate thing about the PS/2 systems is that they lack a hardware reset button of any kind. That can be rather annoying when experimenting with different hardware and software configurations - if the system hangs and it can't process interrupt requests from the keyboard, you won't be able to use the Control+Alt+Delete combo to reboot the system. The only way out is to power-cycle the entire unit, which isn't ideal. It's slow, as you have to wait for the drives to spin down before you re-power the system. And if you have to repeat this procedure over and over, it's not only a big waste of time, but it's not exactly gentle to your hardware either...
Pretty much all personal computers can be modified with a hardware reset button, even if they don't have a provision for one by default. You just have to find a convenient spot for hooking into the reset circuit. This typically isn't too difficult but in most cases it can't be done without soldering, wiring modifications, or other actions that may be beyond one's comfort zone.
One approach, that should work for pretty much all systems, is to add a normally closed button to the Power Good line that goes from the PSU to the motherboard/planar. This signal is typically wired directly to the reset circuit, and if it disappears, the system is put into a reset loop...
Reset on Late Server PS/2s
In case of the late PS/2 systems with the JMP6 - Remote Maintenance Service Connector things are significantly simpler because the JMP6 pin header conveniently exposes the -RESET input. The "minus" prefix indicates that the signal is "active low" - the "action" happens when it's tied to ground (logic "0"). In this case, it means that if we connect pin 3 of JMP6 to ground, the system will reset.
So, all we need is a regular button - ideally, one that already has a pair of standalone pin header connectors attached (for example a power button from any ATX case). Then just wire one end to pin 3 of JMP6 and the other to ground. On the 9595A planar you can take ground from pin 1 or 3 of the nearby J103 - Remote Power-ON header. On the 9585 K/N board, you can use pin "0" of the JMP2 - PAP header.
When you have the button wired in and tested, it's time to find some convenient place to put it. If you have the machine opened most of the time, just hang it off the top side of the DASD cage, or put it to one of the 3.5" bays where it can be accessed even with the side panel installed.
Reset Button Bracket
If you wish to use the switch even with the machine fully assembled, you will have to do some minor modifications. Probably the easiest and least invasive approach is to mount the switch to one of the MCA slot covers. The covers are easy to remove, and you are not risking damage to any of the other parts that may be more difficult to replace, should something go wrong. The exact mounting method will differ depending on the switch type. I would recommend a circular button that comes with a nut and a screw thread cut to its body. All you need to install one of these is a hole of the appropriate diameter...
To avoid unintentional system resets it may be a good idea to use a flat button that sits almost flush with the bracket and one that requires a slightly higher push force to operate (see the pictures for an example).
If the switch you've used has exposed solder contacts like the one on the photos, you should add some heat-shrink tubing or some other insulation over the contacts.
You can ignore the extra two wires on the photos above, these just supply power to the LED inside the button (it has no effect on the reset circuit itself).
I've decided to use Slot 8 for the reset bracket, as it's easy to find without looking, and I usually don't install any adapters to this slot anyway (just so the processor complex is easier to remove/install and to give it some extra airflow).