I too spent the night looking through all 23 pages, and I hope this idea doesn't result in any forehead-slapping.
If the goal is to get the insides of the battery up above 0ºC before charging, what about heating the electrodes directly through conduction into the terminals? These
chassis-mount 8-ohm resistors could be mounted with some heat sink grease directly to the series (or parallel) links between cells. The leads of the resistor are electrically isolated from its body by a layer of ceramic, so they could be run off any voltage available in the pack.
In the case of a 4S LFP pack using three resistors, (one on each inter-cell coupling link,) 12.8 volts would dump 57 watts total heat if the resistors were fed in parallel, with the bulk of that heat dissipated directly into the battery terminals. Some heat would escape from the bodies of the resistors themselves and from the links, but in an insulated enclosure that might still be useful. The resistors are $2.65 each plus shipping.
Affixing a temp probe to the pack's + or - output terminal would detect warming through the battery, so it would give a conservative indication when the internal components warmed above freezing.
Maximizing the effectiveness of this approach might require minimizing heat loss through the wires connected to the positive and negative busses, but if the pack's main fuse was located close to a terminal, that would be suitably inefficient at letting heat pass. In any case, it still might be more efficient than heating the outside of the cell case's insulation.
Another thought: How much internal cell heat is generated by charging/discharging itself? Is is possible some posters who've noted their "batteries hold heat pretty well" have actually been seeing the temp maintained by charge/discharge losses?
I'm contemplating building a four-cell, 12.8-volt 280AH prismatic pack for use in a remote, outdoor location, so I've appreciated reading everyone else's ideas!