You may be able to come up with an estimate by an entirely different means, if you have the right info to hand.
IF - and it's the big IF - Boeing have provided any kind of sensitivity info in your cruise info as to the effect of cg on range/fuel burn, you can use that info to derive the apparent weight due to trim.
Let's say they've told you that every 5% of aft cg movement gains you 1% in range, or fuel burn, or whatever.
Simply work out how much change in mass would give you the same change in range/fuel burn, and bob is at least a close relative - that is pretty much the tailplane trim load.
(I'm ignoring direct tail drag changes, for example, but those are definitely second order for any reasonable tail load).
Apologies if you're doing this to derive the cruise info, obviously it won't be available as a starting point in that case ....