mm43;
It has been suggested to me in a PM that an attempt could have been made to dump the Trim Tank (if it was even thought about!), though a transfer to the center tank would possibly have yielded a better CG result.
Yes - it's possible to dump the trim tank but indirectly: The jettison system removes fuel from the inner and center tanks which would in turn be "replenished" by the trim tank (valve opens on jettison) and outer tanks when the inners reach a certain level until a number of conditions are met which stop the dumping including crew control, the Jettison Gross Weight FMGEC entry.
The CG would move forward as you say. However, moving fuel forward from the trim tank to alter the CG, (the CG is thoroughly monitored by this and other systems and as such there shouldn't be a CG problem in any flight regime), can be accomplished without dumping if such is not required, (and given the dump rate, it is likely to be unhelpful in an emergency). The T.Tank selector switch can be used to OPEN the Trim Tank Transfer valve and all fuel will transfer forward to the center tank. The process of transfer seems to take a while but I've never directly observed/timed it.
In any case, I think we've kind of beat this one up...
PJ2