Happy to report, post-trip, that the Station Landing garage at Wellington is in fact just $5 per day if you park in the MBTA levels (6-8). I was almost surprised when the cashier only asked for $25 when I got back.
There was also another car that I recognized as having been there since early last week (over a week at this point). I have no idea if there is any limit to how long you can park.
Only thing to remember if parking for a long stay - it's cash only. Be sure to bring money as there is only a no-name ATM there if you arrive w/o the cash to bail your car out.