Just to clear:
We have secure webservices. QA needs to test it in near real-world environment.
As every service is secured - it requires signed request.
To sign request I need Saml assertion. Assertion has limited ttl.
So i need regular assertion re-request from identity provider.
My plugin right now filters all requests, finds that request that need to be signed.
Request fresh assertion from IdP, signs request with it, and return it to soapUi.
For tester it is absolutely transparent, it can be used for single or test suite requests.
Just add special tag

and all your requests are signed properly.
Sorry for my poor English.