And, because metro runs on an interval, I don’t have sample accuracy. Snapshot will not reliably give me the zero position – it’ll just give me something close to zero, as it’s dependent on the metro interval, which has no relation to when the sample actually loops. (Hope that makes sense!)

You don't need information from [tabread4~ ], but from [phasor~] to do that. The value of phasor~ increases except for one moment, the moment your new loop starts. So check while comparing each current sample [n] to the one before [n-1] for the moment [n] < [n-1]. You can do that with [expr~ if($v1<$v2, 1, 0)] and do for example the following:

[expr~ if($v1<$v2, 1, 0)]
|
[env~]
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[== 0]
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[change]
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[sel 0]
|
[bng]

Connect [phasor~] to the left inlet of [expr~].
Connect [phasor~] also to [biquad~ 0 0 0 1 0] or if you have zexy to [z~] and connect its outlet to the right inlet of [expr~].

There might be other possibilities but this should work.