Set User Defined Language Programmatically

Question

I've been trying to select the User Defined Language programmatically unsuccessfully through various means, another problem is the files I need to apply the user defined language have no file extension, old DOS files;

NPPM_SETCURRENTLANGTYPE (only enumerates built-in languages)

Macros don't seem to sense changes with language selection, I was hoping to record a macro then trigger it with NPPExec.

Notepad++ accepts only in-built languages for starting from the command line

I can't select a UDL as the default language for a new document.

...and my attempts at overriding an in-built language seem to have failed. I've copied details from userDefinedLang.xml to langs.xml don't work. The highlighting doesn't change.

Thanks for any help!!