Mrs Browl works for a discovery firm; she sugggests that the rolling billing might not work, but getting the foot in the door might allow sales of a long-term cache at a premium price instead eg $10 keeps a page up for 10 years.

Tricky pitch for a new company, of course - you haven't been around so long. Any sort of partnership with an existing one would help. Be aware that law firms in general are conservative about technology so it might take a year or two for the idea to catch on. A way around this might be to target law students first. Contact the editors at prominent 10-20 prominent law journals and give them free accounts for the journal and/or themselves as individuals; let them recommend your freemium service to their fellow students. Law librarians are another likely target, they spend quite a bit of time helping people with research tasks.

I think it would help if these 'permacache links' had some distinctive appearance that was easy to type from a printed page, eg 'http://historio.us/citations/username/98734545.htm <- numeric is easier than mixed alphanumeric if you have to copy it by hand.

In fact you could do 10 digits easily using a telephone format, and then ditch the user id string. 'historio.us/citations/####' is easy enough to become a standard link for public citation, and 111-222-3333 gives you room for 10 billion citations before you need to change the naming schema.