I have not tested this but...
iRule can set a button property for press/release. When this is done, pressing the button sends one code, releasing sends another. The codes for iTach and GC100 relays are discreet on and off codes. So, to make a button and relay combination act like a momentary pushbutton, you set the button for press/release, then drop the on code and off code on it (in that order), and off you go.
You need to trigger two relays with one button press though. Still possible, but you'll have to write your own codes to do it, which is simple. Once you download and install iTach codes, you can look at one, say the Relay 1 On code. You'll see the actual command is a text string: 1,1\\x0D the 1,1 means Relay 1, State 1 (on), and the \\x0D is a return. So, relay 2 on would be 2,1\\x0D (that's a zeroD, by the way). Create a new code under that device and name it R1+R2 On, or the like. Enter both relay on commands with a space between like this:
Now you have a code that turns on two relays. Do the same to create the off command code:
Note the commands are similar, but the relay is now set to state 0 (off).
Set a button for press/release, and drop the two new codes on it, on first. Now you have a momentary button that turns on R1 and R2 when pressed, and turns them off when released.
The commands could be anything. Heck, you could turn all your relays on and off simultaneously that way! Or create combination codes and with only 3 relays end up with 7 possible outputs, binary encoded. You could, with a single IC decoder chip, control 6 different devices even though your iTach has only 3 relays. Ok, that might just be too much excitement.
Again, I haven't tested this, but I've written enough codes to think it will work fine.