So I'm thinking about this a little bit more, especially through the framework I'm trying to embrace of "phased solutions" -- i.e. breaking projects up into smaller pieces, each of which is a reasonable solution suitable for some amount of time, though not necessarily sustainable in the long-term.
I still think that an LRT shuttle is the wrong way to go. However, I wonder if an additional short-turn commuter rail shuttle service could be used to lay the groundwork for public support for rapid transit expansion (on either side of 128). Keep the current hourly trips into South Station, but layer in short-turn trips to Forest Hills every 15-20 minutes, with free transfers to the Orange Line. (In fact, in general, you should probably move the line to rapid transit pricing anyway -- maybe with a small surcharge if you ride "express" to South Station, but I don't think that's worth fussing over.)
That would start to offer some of the benefits of rapid transit service without the need for huge upfront capital investments, and without sacrificing the quality of the current service. Aside from needing some extra equipment, this is a change that could be implemented purely organizationally, without new infrastructure.*
* The one part I'm less confident on is that the majority of the branch is single tracked. There are passing sidings
- just inbound of Needham Heights
- just inbound of Needham Junction
- a bit outbound of West Roxbury
- about three-quarters of the stretch between Roslindale Village and Forest Hills is doubled -- but, there are no crossovers outbound of Forest Hills, so a short-turn would need to reverse out the same track it enters on, and one of the tracks merges onto the mainline outbound of Forest Hills, so that's not really an option
And it's that stretch between West Roxbury and Forest Hills (all effectively single tracked) that would really pose a problem. 10 minutes from W Rox to FH, then x minutes to turn, then 10 minutes from FH out to W Rox. Assuming x = 10 minutes, which is aggressive, then you're at 30 minutes between inbound trains. Now, with a free transfer to the Orange Line, that's still probably an improvement, but not a huge one.
That being said: a modest capital project would be to add a second track between Roslindale Village and Bellevue (leaving the stations untouched). Assuming that this could be done in a way that it wouldn't need to be redone for an Orange Line conversion (i.e. lay down a second commuter rail track and use that same track for the Orange Line when its time comes), that could probably get you down to a 20 minute headway (depending on how fast they can turn the train at Forest Hills). This would provide a tangible improvement in service, and would lay the groundwork (physically and politically) for rapid transit conversion.
EDIT: But in case it wasn't clear, the long game and best option is an Orange Line extension.