This implements Ivan's suggestion:
This is actually how chain rigs work in my own Rigify featureset, so as far as I'm concerned this is a tried and viable solution.
The result is that, if you want to parent something to your chain rig, you can do it in two ways: If you parent it to the ORG bone(ie. by default), it will not inherit any squash and stretch induced by the "tweak" bones. If you parent it to the DEF bone however, it will behave the same as before.
This raises a backwards compatibility concern; If you want the old behaviour, you have to express that in your metarig explicitly, by parenting your bone to the DEF bone in some way.
This patch also only affects the spine rig, which makes it inconsistent with other chain rigs in Rigify in this regard.
If this patch is option A, option B is to make this change in the TweakChainRig class, which would affect every chain rig in Rigify, and the aforementioned backwards compatibility issue will apply to all other rigs as well. I assumed what's in this patch would be the preferred solution, but personally I actually prefer this solution, so if you wish I can change this patch to fix it in this way.
Option C is to simply lock the neck bone's translation and consider it not a supported feature.