I've had a look over it mate and I don't think it would be the best way to do it to adapt this board. To get the drains within standard trimmer distance of the supply rail, avoid affecting other components linking on the rows between drain and supply, and adding enough width to physically fit the trimmers on the board would end up adding a good few rows and columns. It would be much more manageable, and easy to actually perform the biasing if we added a little separate daughterboard for the trimmers. The bonus being that it could be mounted away from the main board, or even float above it and so not add anything else to the size of the main board. A length of ribbon cable or similar to link to supply and all the drains is all that would be needed and then something like this for the daughterboard:
I may do a new layout including trimmers because if I'm accommodating them from the start I can do things a different way rather than trying to adapt an existing layout.
Alternatively if you use the trimmers with long legs that you can snip to size like the Alps RH06, you could just mount the trimmers in place of the resistors with no mods at all.