The Loctite people seem to have a couple of products for this. They use a primer (Loctite 770 Prism Primer) with their adhesive, 401 Prism.
Do I picture this correctly: you are milling the edge away a square cross-section off the edge, so that when you lay a square rod of replacement Delrin on the edge, it just replaces what you milled away? Would there be a cost-effective way of doing this with dovetails so that you just slide a properly shaped rod into place and the dovetails hold it? Might be worth it, if you want the mend to be invisible.
They make acetal screws, btw.
See here:
https://www.ellsworth.com/globalass...nkel-loctite-design-guide-plastic-bonding.pdf