"The major disadvantage from the Anglo/Duet handstrap is that it locks the fingers"

Not so with the Anglo. It's all down to good technique.

If you play with your fingers pointing forwards (your elbows at roughly ninety degrees), then I'd agree, as the weight of the instrument will compress your fingers together.

However, if you have your fingers pointing upwards (your elbows at roughly twenty degrees), you will find that the weight distribution is radically changed, your fingers are freed up and you have no problems.

If you doubt me, watch John Kirkpatrick at work. It's the way I play, too...