Well, with the trimpots mounted where the resistors are, it may be unsafe or inadvisable to do too much poking around while playing.
A simple way to go about this is to get some rca y-adapters, such as these:
http://www.partsconnexion.com/BB-67500-086.html which are on sale from partsconnexion. If you remove the resistors completely, then run the trimpot from one of the male center pins to the outer part of the rca y, you can plug in the phono leads to the open female end of the y (with the second male end plugged into the seduction) and then very easily adjust the loading. I don't think there's any reason to not do this on the fly, but perhaps someone else can chime in.
Just make sure you have the trimpot connected correctly so that when you measure resistance between the female center and outside, you see that resistance. I don't know the effect of having a completely unloaded or shorted cartridge, but neither sounds like a great idea.
Once I have some time, my intention is to take this a step further and just make a little box that plugs into the y-connector, which has a stereo pot in it, for easy adjustment of both channels at once.
As far as the type of trimpot, I just grabbed the cheapest ones from the local electronics store which covered the range I was interested in. As I wasn't going to leave them in long enough to do anything but find which setting was the best, I didn't think I needed to worry about their quality.
I hope all of this is clear! Let me know if I've confused you, and I can put together a couple self-explanatory little pictures.
