@MTNathan, turns out there appears to be an undocumented method

nlapiSetFieldLabel(<field name>, <field label>)

that does exactly what I’m looking for.

Interesting, I guess that's one of those cases where SS1.0 is a better option than SS2.0 (as long as you're comfortable with undocumented functions). Definitely better than DOM manipulation.

Agreed. But undocumented functions are such a PITA

For sure, especially when they're exactly what you want and there's no documented/officially supported alternative