I’ve seen other CMS’s using a custom protocol scheme for marking internal links, e.g.:
kirby://some/file.pdf, which seems to work quite fine. One caveat: The field content woudl hav to be parsed before being useful.
Another possibility could be the usage of custom attributes, e.g.
<a href="/some/page" kirby-link="i324-ohui45h3u…">. The latter approach would probably work much better once Kirby supports persistent IDs, because it remains transparent what the link should point to, while still allowing Kirby to generate an absolute URL at runtime based on current domain, language etc.