Pete, although I don't use one myself, I'm pretty certain a sensi based system could be made to be extremely effective. However, to develop it for just yourself is probably nowhere near cost effective. I doubt a spreadsheet based program would be useful enough.(posted 8732 days ago)Do you already own the densitometer? If not, I would point out one real problem; most machines will not allow you to read into really small areas. Something like a 3mm (transmission) diameter reading spot is typical. I know that Macbeth units can typically be fitted with either 1 or 2mm diameter apertures; even so, this may not be small enough for a typical 35mm film user. The black mask around the reading aperture makes it hard to accurately position your neg.
Although I said that I don't personally use such a system, I've had a lot of experience in these general areas; feel free to email with more specific questions if you'd like. IMO, you are exactly on track on the part about translating subject brightness into paper density.