I have a 2-vessel RIMS system with a bottom-draining keg for a mash tun. I run everything through a RIMS tube fired by a PID. If I understand your design correctly, you want to manually control your heating element AFTER your controller to ramp up the temps. I'm not being a naysayer, but I see a few things you might consider. First, the 2352 PID from Auberins allows bumpless switching from automatic to manual. You could theorectically use that function for ramping. I've messed with it a little, but not enough to really test the efficiency. Secondly, instead of a rheostat (dimmer), you might want to look at pulse width modulator (PWM). There are several threads here if you just search on PWM. Many advantages over a rheostat. Lastly, placing your heating element in the mash tun, based on my experience, is not a very good option. A considerable amount of grist makes it's way through the false bottom in a keg mash tun. Having that concentrated slurry in direct contact with the element, even for a very short time, risks scorching. Anyway, just a couple of suggestions you might want to consider. Good luck.