Close reading https://standards.scheme.org/official/r6rs-lib.pdf page
26, it says:

"(condition-predicate rtd) procedure"

"Rtd must be a record-type descriptor of a subtype of &condition. The
condition-predicate procedure returns a procedure that takes one
argument. This procedure returns #t if its argument is a condition of
the condition type represented by rtd, i.e., if it is either a simple
condition of that record type (or one of its subtypes) or a compound
conditition with such a simple condition as one of its components, and
#f otherwise."

I take this to imply that wherever the standard says condition type &foo
is raised, the implementation can actually raise any compound condition
that has a simple condition of type &foo as a member.

So implementations that add R6RS support could keep their existing
native exception hierarchy, and make those exceptions use
R6RS-compatible compound condition objects with R6RS-compatible simple
conditions as members. This should solve all interop problems?