I've been wishing I had a swivel on my Lock 'N Lube coupler that is attached to a Milwaukee M18 grease gun but it generates 10,000 PSI and most swivels are only rated for 3500-4000 PSI.
Lock 'N Lube just came out with a new swivel that is rated for 10,000 PSI. It is model number LNL231 ($10.99).
I've been using the LNL with the swivel for a few weeks now and it really makes getting into tight places easier. I even gave it the torture test which is greasing the front axle pivot on my 2720 as the tolerance is so tight that it seems like the grease gun hose will explode before the joint starts taking grease. It built up lots of pressure and the swivel did just fine.