After trying out some switching transistors, I settled for 2SJ200 - a lovely, though somewhat expensive switching P-FET. I made this choice mainly due to its very low switching treshold - it is important we achieve saturation when switching 3.3v with 3.3v control signal (refer to Vds and Vgs on 1st figure on page 3 of datasheet). Power-wise, this FET is significantly over the top, but it is the best one that works and is readily available at this point. Later down the road we may replace it with a cheaper one.

It is very important that for this particular application, P-FET be used as arduino signal pins can (and will!) ground. Since positive supply line is common, it only makes sense to put the switch there.

Initial tests have shown no problems, and a long-term test is already underway. Look forward to pretty pictures!