Assuming there isn't any flexibility on the contract, there are two possible answers here: The "proper" answer and the "real world answer".
The proper answer is that of course you can't write the technical specifications before you do the research.
The real world answer is that there are often times where a contract is not getting signed without technical specifications, and nobody really cares what they are and will probably never read them again.
This is not a UX problem; it's a business problem or perhaps a management problem.
However, assuming that you're being forced to provide technical specs and that refusing is not an option then do some of the research ahead of time and hope you get the contract.
If you absolutely can't, then:
- Try to be as general as possible
- Use lots of phrases that sound good but don't really commit you to anything, especially if you can mirror phrases from their RFP ("Provider will deliver an easy to use, well documented, and highly available content management system")