Does a glider pilot need to file an IFR flight plan to legally fly in cloud in class E airspace in the US? Does a glider pilot need to file an IFR flight plan to legally fly in cloud in class E (controlled) airspace in the US? Or could he or she contact ATC and receive a clearance to enter cloud without actually filing an IFR flight plan?

Assume that the glider pilot has an instrument rating in airplanes (as required by FAR 61.e.3), and the glider has all the equipment required to legally operate in Instrument Meteorological Conditions under Instrument Flight Rules.  

Background information-- I know of a case several decades ago where a glider pilot legally thermalled high into a towering cumulus cloud to set a state altitude record under conditions similar to those described above.  The state record would not have been granted if the flight had been conducted in violation of any FARs.  I don't know whether or not an IFR flight plan was actually filed.  Note that FARs 91.619 and 91.153 do contain the phrase "Unless otherwise authorized by ATC" when describing what information must be included on in IFR flight plan, so if an IFR flight plan was filed in this case, it may have looked quite different from a typical one.

Related-- 

Does a request for a Pop-Up IFR clearance constitute filing a flight plan?

Can you file an open-ended IFR flight plan? 

What IFR clearances may a pilot obtain from ATC directly without a pre-filed flight plan with FSS?

What IFR clearances may a pilot obtain from ATC directly without a pre-filed flight plan with FSS?
As answered here, requesting a pop-up IFR from ATC counts as "filing" a flight plan.

It should be noted, as explained here, that airfiling with ATC is a "workload permitting" activity, meaning ATC is not obligated to grant it. They can always refer you to FSS.

Climbing or descending through a cloud layer is one of the generally accepted use cases for pop-up IFR requests, so while ATC might be surprised by a glider asking for it, I can't see why they wouldn't treat it the same as an airplane requesting the same.

The main challenge I see, at least for non-motor gliders wanting to climb, would be phrasing the request correctly to get a clearance they can physically comply with. Maybe "circling within Xnm of present position" to allow for thermaling? It's also possible to request a 3D box if you want to do more complicated maneuvers/work; I'm pretty sure I've seen a question here on that.