kombi, this exact thing is also very high on my list of things to do! Good luck with it - and let us know how things turn out.
With regard to that 215 gr, round nose - did you know that the 220 gr round nose (developed for the 30/40 Krag) is the 30 caliber version of the famous 500 gr Government round nose for the 45/70? Your 215 gr bullet should be very similar, and that's the bullet I would use for loading the 303 with BP.
As far as cleaning the bore - water is the key. Solvents developed for somkeless won't work. BP fouling is water soluble, and that's all you need. I use a mix of 50% water, and 50% Prestone antifreeze, with a little Murphy's oil soap - just enough to act as a surfactant.
The antifreeze is, in essence, a water-soluble oil, and I feel that if you miss some spots when finishing up your cleaning, the antifreeze will help protect the bore (this formula comes from one of Paul Matthews' books). I don't even bother oiling the bore after cleaning, but here in Montana it is very dry and rust is almost a non-problem. In a humid climate I would definitely oil the bore after cleaning.
I have not heard much from people shooting BP in the 30 cal cartridges. It seems to be somewhat unknown territory. Good luck, and please keep us informed!