trying to play around with pants in a repo that has existing BUILD.* files; is there a way to tell pants to ignore certain BUILD.ext extensions?

Yup:

$  ./pants --help-advanced | grep -A3 build-ignore | head -4
--build-ignore=<str> (--build-ignore=<str>) ..., --build-ignore="[<str>, <str>, ...]", --build-ignore="+[<str>, <str>, ...]" (@fromfile value supported) (default: ['.*/','/dist/','bower_components/','node_modules/','*.egg-info/'])
    Paths to ignore when identifying BUILD files. This does not affect any other
    filesystem operations. Patterns use the gitignore pattern syntax (https
    ://git-scm.com/docs/gitignore).

To keep the existing ignores and add your own, the syntax would be (assuming you want to set this option for the repo and not adhoc):

ini
[GLOBAL]
build_ignore: +['BUILD.ext']

Oooh, that’s gonna be useful (I have codegen directories with stub BUILD files I want to ignore)