SLA Agreement Selection Priority: Is there any way of adding to the list of fields available?

I’m not sure if I’m missing something obvious here, or we’re just trying to do something that isn’t possible, but here is our scenario:

Our call centre staff take a call from an external customer.  They use an InfoPath form to go through a set of questions about the issue, which includes specifying what service level the external customer is on.  (We use InfoPath as it gives us much more flexibility than using the basic VSM portal forms for logging a call).  At the end of the form it sends an email to one of the mailboxes polled by VSM, based on the service level selected on the form.

Each mailbox polled has a different Call Template applied to it.  We have added the standard “Service Level” field to the call screen and each call template has this field set to a value matching the service level specified on the InfoPath form.  The details of the external customer and the issue details are put into the call Description field from the emails body.

We want to automatically apply different SLAs to these calls; however, you don’t seem to be able to specify the “Service Level” of the call in the SLA selection priority, only the "Service Level" of the Customer, Organisation, Service or CI.

We have to leave the call Customer as the call centre operator, so can’t change this field to be a Customer record for the external customer and base the selection priority on this.  The call could relate to various Services and CI’s, so we can’t base the selection priority on these either.  We can’t base it on Priority, as each service level of external customer could log different priority calls.

An example: an external customer classified as having a Service Level of Tier 1 has a call logged for them; we’d want the “Tier 1” Service Level SLA applied to the call based on the Tier 1 “Service Level” value selected on the call when it is logged via email.

I hope that made some sense?  Does anyone else have a similar situation to this and know a way of getting this to work?  Or is there a better way to go about doing this that I am missing?

Many thanks for any advice,

Ian