Is there any publicly available research that projects pilots' loss of proficiency due to lack of flying (or guidelines to prevent it)?
Any decent accident investigation report will show the flight hours flown by the pilot/crew, broken into a matrix of all / acft type in question, and during certain periods of time. These reports are publicly available on different national investigation authorities sites (for example NTSB). The reports also evaluate the currency of the pilot/crew, and mention if it is considered to be a factor in the accident.
If you are looking for an easy way out, in form of a scientific study on these reports, sorry, can't help with that one.
As mentioned in comments, currency in flying does not equal proficiency, and you seem to understand this. Being current does not guarantee one is proficient, not by a long shot, so you are digging a real can of worms here, beware :)
I do think you would benefit reading up on pilot training literature, namely the &quot;human performance &amp; limitation&quot; module. IIRC it contains some very relevant information on the subject of your interest. Such as how experience needs support of other positive personality traits to manifest itself as safety, and perhaps proficiency if you will.