Does lightweight pooling disable built-in CLR facilities?
Yes.
When running in fibre mode, everything that needs the CLR is unavailable.
For example, both:
SELECT FORMAT(GETDATE(), N'd', N'en-us');

and:
DECLARE @g geometry;
SELECT @g;

...return the (slightly misleading) error message:

Msg 5846, Level 16, State 2, Line 13
Common language runtime (CLR) execution is not supported under lightweight pooling.
Disable one of two options: &quot;clr enabled&quot; or &quot;lightweight pooling&quot;.

It is the lightweight pooling that is the problem, no setting of the clr enabled option will allow CLR execution of any kind under lightweight pooling.