Yes, a Surgeon knot with doubled-over tippet would form a nice loop but it would not tighten when pressure is applied. My search is for a loop that will tighten once pressure is applied as is the case with the Duncan Loop .
Regarding your comment:
.."Just another thought - if the knot is generally recognized as the weakest point of the leader, does it increase strength any if you go from a single strand to a double strand with a knot? Not being smart, just asking..."
Part of what determines knot strength is whether or not the line comprising the knot ends up cutting into itself (or getting "cut" by another larger diameter line it's being attached to).
Take a length of 20# tippet and blood-knot it to a length of 10# test. With just a bit of pressure you can break the knot quite easily because the 10# test is in effect "cut" by the 20 # test.
Take the same 10 # test tippet length and double it over. Tie the same blood knot to the 20 # length, tighten carefully, then trim the resulting ends. If you're using a burly line like Maxima it'll cut your fingers before it breaks. Same concept when you tie a knot like an improved clinch knot with doubled-over line. This is a wonderful knot to attach a fly to a leader. There is no question the doubled-over tippet adds knot strength. But this knot can't be used so as to form a "loop". So I continue my search. Thanks for the comments and suggestions.