The St Pancras (East Midlands Trains) lounge staff have also confirmed to me that they have been instructed to allow East Coast passengers to use the St Pancras lounge while the Kings Cross lounge is closed for refurbishment. They did not indicate, unlike the VT staff at the Euston lounge, that any time restrictions apply.
This was also the first time I had used the relocated Midland Main Line St Pancras first class lounge. I was surprised to find - particularly when, as I recall, the first class lounge at Sheffield is very nice - that it is still small (only very slightly larger, if that, than the old lounge which was located closer to the Renaissance Hotel / further from the MML platforms), and looks shabby and forlorn (worse than the old lounge, actually), which is made all the more stark by the comparative beauty and grandeur of St Pancras station itself. The staff were friendly and helpful, but to not even have a coffee machine in a first class lounge at a key London terminus is unimpressive on East Midlands Trains' part. It is true that EMT's range of first class tickets (except season tickets) is cheaper than that offered by (in decreasing order of price) Virgin Trains West Coast, Virgin Trains East Coast, and Great Western Railway (except season tickets for which Great Western Railway is by far the most expensive), so I wasn't expecting the lounge to be up to the same standard as Euston, Kings Cross or Paddington, but it was still slightly disappointing.
Speaking of Euston, while the first class lounge is excellent in most respects (providing the air conditioning is turned on and there is no music playing), it's key shortcoming is that it is too small, and so I was disappointed to discover that Virgin will not be taking advantage of the newly constructed balcony at Euston station to extend the size of the lounge.