Customize OpenIddict client credentials validation

Question

How does one customize the client credentials validation in OpenIddict client credentials flow, e.g. if you had grant_type as client_credentials and client_assertion_type is urn:ietf:params:oauth:client-assertion-type:jwt-bearer and wanted to apply some custom validation for a standard such as FHIR back-end auth. This standard has a slightly different set of requirements than the base validation built into OpenIddict, e.g. the iat claim is not required [EDIT: fixed in 5.1.0], and the JWKS rules are slightly different.

I see that you can supply a different JsonWebTokenHandler ...

options.Configure( o =>
{
    o.JsonWebTokenHandler = new MySpecialTokenHandler();
} );

... but this doesn't have any other context about the overall request.