Impersonation AccessControlPolicy question

[Robert Führicht]

Dear all,

I was asked to allow impersonation of (relatively) arbitrary usernames
with IDPv4 for a particular in-house SP.

A group of users (support staff for SP) should be able to impersonate a
number of user accounts (around 30K). The user names are easily
addressable with a (short) regular expression.

I set up impersonation as documented here
(https://wiki.shibboleth.net/confluence/display/IDP4/ImpersonateInterceptConfiguration)
and it works - but only with a list of usernames.

I'd like the form presented in the intercept flow to use a textbox
(rather than a dropdown from the example) and have the input validated
against a regex (rather than a list of usernames) read from an attribute
(e.g. impersonatableUsernameRegex, to stick closely to the example).

Is there a better way? Is this possible?

Kind regards,
Robert