Recently, though several of my clients have taken a close look at this and have started to address this issue head-on with a simple solution.
What they are doing is making a simple addition to their initial RFP's. Along with the standard meeting dates, room block, F&B requirements, etc, they now attach their own form, contract or addendum stating their company's terms for acceptance of the RFP. Included in that document is a section concerning audiovisual and IT which states that they reserve the right to choose their AV and IT providers without restriction or penalty
The results so far are very positive. We are able to design AV packages that are better matched to the needs of the event and at the same time able to better manage the overall AV budget. It's a win-win. The only catch is that this must be implemented at the very beginning of the RFP process, so that the venue knows the terms of acceptance up front. Once the contract is signed however, there is no longer any bargaining power.
If more companies were to fully realize the degree of control they have during the initial RFP process, to adopt this procedure and to clearly state their terms for acceptance up front, the end result would be better events and fewer budget surprises