re: getting an air hose on the front value stem of the C650GT...
It certainly would have been nice for BMW to use a 90 degree valve stem on both front and back, but that's just a dream.
I've tried a few after market 90 degree valve extensions with marginal success. They seem to be prone to air leak over time. I suppose you could pull the tire and replace the valve stem with a 90 degree valve stem, and when I have the tires replaced, I will ask the shop to do that.
So, as of today, I still struggle (unsuccessfully) to fill the front tire. I have a 10 gallon compressor at home with the standard attachments, but none work well.
On a side note, I don't believe the tire pressure readout on the dash corresponds exactly to the actual tire pressure of the tire. When I fill the tire to 40 pounds, the TPM/dash will show 34 or 36. Is this true of all TPMs or, just a fluke? (Readings are taken with cold tires).
========================== Update ==================================
I purchased the 45 degree valve extensions pictured below (eBay) and tried to install them and leave them on the bike. They would unscrew a bit all by themselves and leak air (as I mentioned above in my earlier post). So now what I do is put them on, then attach the pump/compressor and inflate the tire, then remove them. Although it still isn't the easiest thing to do, it does work.