I've added another flag variable that allows me to turn off the ban on implicit loading in dump mode.
The dump with all ~2500 ELN libraries loaded completed last night. Unfortunately during startup (before any frame opens) one of them is evaluating an _expression_ with an argument that should never produce nil but somehow is nil.
Fortunately I can now redump with just the offending library in site-load. The error prints fairly quickly when run normally, but shared libraries evidently open a lot slower under GDB.
Hopefully I'll get the same error with the smaller dump and be able to determine if there's an issue in the dump file, the dump loading, or something else.