"how to calculate maximum combined temperature coefficient and drift, given specs of an LM741 and the trimmer pot."
That's easy. You can't. You need to pay more attention to both your parts and the data sheet. Look closely at your 741s. What part number? I'll lay very good money that they are LM741C. Now look at the data sheet. See where it says "Average Input Offset Voltage Drift"? Check the right-hand column, under 741C. Notice anything missing? Like a number? That's because there is no spec for offset drift for the 741C. Likewise check out the offset current drift. Nothing.
So, even if by some miracle you have 741s instead of 741Cs, just try to find a max number for either of these quantities. You can't. They simply are not specified, so you take what you get.
Your underlying problem is that, as the first cheap op amp which required no compensation, the 741 was in no position to worry unduly about small temperature effects. There's an old saying, "Faster, cheaper, better - pick any one." and this applies to the cheaper 741. As a counter-example, consider the OP27, which came out a few years later. Go to page 14 and you'll find a fairly detailed discussion of offset/temperature adjustment. The early days of op amp production were marked by major advances in die processing and laser trimming, so it didn't take long to leave the 741 in the dust.
I can understand your attachment to an old favorite, but "it's hard finding one which is as cheap and versatile enough in the ways a 741 is" is just wrong. The 741 isn't versatile (it absolutely requires +/- 15 volts, although you can usually get away with +/- 12), and it isn't all THAT cheap. For instance, on eBay you can get OP27s for the same cost as 741s. Oddly, 741s are a good deal cheaper at Digikey than on eBay, but on a per-amplifier basis something as old as the TL084 is very nearly the same. And a 084 has 4 orders of magnitude lower input currents and can run down to +/- 3 volts. Plus, of course, it's got 4 amps per package.