It's many years since I flew 737-300s and -400s but I remember doing a similar thing. I think one had to display the POS page that shows the individual IRS positions as Lat and Long, line select one of them (bringing that position into the scratch pad) then switch to the LEGS page and line select the scratch pad position into 1L, making it the modified active waypoint. If the EHSI was on minimum scale, one could see the waypoint star moving away, behind the aircraft.
Three obvious points:
1. Don't execute, otherwise the aircraft is going to do a 180 towards the new active waypoint which will be behind you by now!
2. All the above has to be done swiftly, as the line selected IRS position is 'frozen'.
3. The same check can be done by viewing the 'POS SHIFT' page, where the IRS positions are displayed as a bearing and distance from the FMC position, without any time delay.
I don't think that the radio position could be line selected or displayed.
The FMC position is always at the top of the triangle at the bottom of the EHSI, so that is always displayed anyway.
Hope this helps (my memory cells are decaying rapidly!)