Servos are made in various build qualities. Some have a ball bearing (and some have two) on the axle at the business end and they can take a good bit more than those with just a brass (or even plastic) bushing of course, but most come in a plastic house, so even with bearings, there is a limit to what it will stand.
I don't think there's any guides about that, since it's an unconventional use of a servo, but if you know all the data (about bearing/bushing and housing), it can be calculated, but I guess it will be really hard to come by the needed data.
Lead/acid batteries comes in a lot of different sizes as well, so instead of us guessing, tell us the total maximum weight.
Even without knowing the precise weight, I think it sounds like you'd be destroying your servos (if they can drag all that weight around at all).
When lead/acid is involved, a DC motor and axles of (at least ~6mm) in diameter sounds more useable.