In fairness to Brooks, I am sure they got sick of warranty claims for saddles that were ruined with some sort of DIY goop. Anyway, they are careful to say that other products will jeopardize your warranty rights, not null them.
BROOKS ENGLAND LTD. | FAQS | SADDLE_MAINTENANCE
They do specifically say not to use neatsfoot oil.
I'd think if waterproofing is the main concern, a bit of ordinary paste wax is a pretty safe bet. (ie Johnson in the yellow can).