Howard:"On an anglo, thumb straps would inhibit the use of the air-button and left-hand drone button (where fitted)".

RE:No problem at all to fix that! I have done it on mine by just re-locating the hand rest/bar a little to free the air-button. It works even better by using levers acting sideways ( similar construction as "bowing-valves" on Englishes) instead of press buttons which are not suitable anyway for the thumbs since the distant joint of the thumb does not act in that direction (this has been practised by Geoff Crabb on his instruments)

H:"You regard "static work by the thumb" as a disadvantage. I find it is helpful to keep the straps slack enough to let my hand move sufficiently to reach different fingering positions".

RE: That is a dysfunctional way dealing with the situation.Avoiding static work is a general ergonomic primary object.The use of a thumbstrap offers a passive stable pivot point for the rest of the hand to slide reaching other positions. Do you have any experience from playing English? then you would know...

H:" The thumb can be then be used to tension the strap when required. If my hand were more firmly fixed in place it would restrict my playing".

RE: With an Anglo and a suitable position of the handle most individuals can manage 30 Anglo keys with a fixed hand and that is the best when possible since instrument control can only benefit from better stability. The individual length of the little finger may be a limiting factor reaching the most distant buttons on 3rd row for sure but a higher rest located closer to the keyboard at the little finger side deals with that.