Hi Charlie,
It sounds like a good idea.
For us, we would perhaps want to advertise in various Towns and Cities for commercial work only.
Count us in, and let us know further details as and when they become available.
For Commercial Window Cleaners, perhaps the site could only accept companies who attach/download certificates of their Insurance, Safe Contractor Plus or CHAS, even as far as adding in skills/Qualifications such as IPAF, PASMA, NVQ CSCS - this would help larger companies such as yours easily assess which potential sub contractors to approach.
I think there are too many sites advertising anybody who wishes to pay and not enough who source/showcase the true professionals out there.
We are about to offer something similar ourselves to local window cleaners who clean domestic windows only, as we often get requests from many domestic customers from all over the place.
It would be free to add a link, providing they added our link to their site also. Our new site should be up and running quite soon.
Good luck.