Ok, turns out I had some more time than expected. The branch of archie in Opt 1.4 parser and converter by pieterbos · Pull Request #261 · openEHR/archie · GitHub now actually converts at least some 1.4 OPTs to a working ADL 2 template.
The resulting template is int differential source form, which can relatively easily be edited by hand or with tools. It can be converted to OPT 2 easily as well with the existing Archie tools, or the visual studio code extension.
This is still very experimental, but might be worth a try. Even just loading an OPT 1.4 into AOM 2 and to use it from there with the at-style nodes and adl 1.4 semantics is possible, and path lookup will work.