With Derek's general set-up--eye in end of hitch, oriented to pull tangentially--,
one should be able to control where the hitch is, by holding in place this eye
as the hitched part is drawn through the knot, or pulling/releasing the eye to
whichever direction seems needed. However, so long as predominantly the line
is drawn through a stationary knot, that line is moving ONE way through the
entire and increasingly tight-upon-it hem, and will drawn the hem with it.
Maybe there is a strong force, thus, for the hem to arrest this drawn-end movement
and coerce compensating movement of the knot, though I'd think that one could
position one's hold on this so as to at least become aware when this force occurred.
But one cannot have the two
ends (i.e., knotted end & hitched end) make
equal movements in opposite directions (so as to equalize the line's draw through
the halves of the hem), because the knot is around the other line and would
have to move into the hem whatever distance was needed to balance.
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There is a (theoretical

) way to, er, *
centralize* things, so try this tactic:
1) size your cord so that it wraps around the desired circumference TWICE and more;
2) form a Prusik Hitch around air in the center, and bring each end, in opposite
direcitons through the hem, around through this hitch;
3) work the hitch snug/tight;
4) pull ends in opposite ways to tighten evenly (in theory--that's all I do here!)
In this set-up, having the doubled cord through the hem might help in some
way diminish the friction against it? In any case, the cord will be moving equally
in both directions, so the hem should be *unruffled*.
As for ease of untying, well, c'mon, ain't 2 outta 3 okay? Is this g'mnt work?
One could get extra clever (pain on the tying end) and arrange to make this set-up
in reverse, beginning with a loop to cross the ends (which loop will be enlarged
in pulling it wide to tighten...), run ends through hem and each emerges to tie
one half of the Prusik, being themselves tied off (Reef Bend) to form the Prusik's
cross-part (I've seen it recommended to orient the typical Grapevine bend
in a climber's/caver's "prusik sling" positioned here so as to keep the knot from
jamming so much (YMMV))! That's maybe too clever by half, but, hey, it's only
keystrokes for me!

The above, yes, entails more cord.
But one expects to suffer the usage of this, so if it's a better mousetrap in that,
it might be worth it. (Btw, the idea of cost of these fiddly bits being relevant
surprises me--it just isn't all that much cordage!)
--dl*
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