Several clients have the need to bill based on costs, but still need an AIA format. For example, I have many customers that have a schedule of values where the line items' SOV is based on cost. They then add one or more lines for Contractor Fee, Profit, etc.
If the AIA was available for the T&M billing, you could write a billing rule that calculated cost incurred for each SOV Task and then add a formula for the Contractor Fee SOV line that would calculate how much Contractor Fee was earned that billing period.
The other option is to be able to have the Substantiated Billing process complete the Pending Invoice Amount on the Revenue Budget tab.