It should be supported.
The "standard" is here: https://authindicators.github.io/rfc-brand-indicators-for-message-identification/
It says only this:
l= locations (URI; REQUIRED). The value of this tag is a comma separated list of base URLs representing the location of the brand indicator files. All clients MUST support use of at least 2 location URIs, used in order. Clients MAY support more locations. The supported transport is HTTPS only.
SVG format is required however.
Note however that you also have this:
The receiver may use a manually maintained list of large brands, or it may import a list from a third party of good domains, or it may apply its own reputation heuristics before deciding whether or not to load the BIMI indicator.
But having resources outside of main domain is kind of hinted by the following:
9.4. Unaligned indicators and asserting domains
There is no guarantee that a group responsible for managing brand indicators will have access to put these indicators directly in any specific location of a domain, and requiring that indicators live on the asserted domain is too high a bar. Additionally, letting a brand have indicator locations outside its domain may be desirable so that someone sending legitimate authenticated email on the Domain Owner’s behalf can manage and set selectors as an authorized third party without requiring access to the Domain Owner’s DNS or web services.