Well as a planner, resident of McLean and someone who has to frequently use those handicapped parking spaces, signage is only part of your problem.
1) Talk to FFX County and see what the rules are on the HC parking space itself, if you are doing much structural you will probably have to bring the parking lot up to code.
2) Talk to FFx about signage they are going to require you to have the lovely blue international symbol of access. I am not up on my international building code but I think its in there.
3) Find out what dealings you may have with the McLean citizens association, do not discount this body they hold a lot of oomph when they are against a project.
Now personally as a user of those spaces exactly what do you find obtrusive? When someone is handicapped you become used to looking for that sign, its as the name states UNIVERSAL, not being able to find it because its less visibly obtrusive is going to be a problem. Your client wants business yes? In an area with a lot of handicapped persons it seems counter productive to make it harder for ones clients to access a site.
Oops just took the time to Google up the regs and this is it
http://www.fairfaxcounty.gov/DSB/handipark.htm#signs you do have to use the signage that is probably more obtrusive than you are wanting. Google is your friend as most jurisdictions in this area have everything you need right on the web.
For what its worth I do not work for FFX County and have no association with the McLean Citizens Association.