Your caution is commendable, but if the charge is thoroughly wet from both sides, the danger should be minimal. Exposure while pulling the ball is about equal to loading if a fixed puller (something to hold the ramrod) is used, NOT another person pulling on the ramrod. Anyway, I don't think it is rash if done carefully, if your conscience can be swayed, but I won't try to convince you any more forcibly.
Otherwise, how about a grease fitting in place of the nipple, then pump out with grease gun. On problem with "vintage" muzzleloaders is that they often have somewhat rough bores (which compounds stuck balls) until they are either polished a little or shot some (not to mention the PYRODEX, gasp! -- that is another can of worms), so the grease won't do any harm except make a mess.
Good luck, and try to check the thing over carefully. If it was shot with Pyrodex and not cleaned properly every time, it could be badly corroded, not just the bore, but also possibly the breech.