I don't know if this would work because I've never used "Keys". There are utility programs out there that let you create "macros" that you can assign to specific keys. These "macros" can be a combination of keystrokes and mouse clicks.
So, you could install "Keys" and one of these macro utility programs, then use the macro program to create macros that select a particular pushpin, with the last step in each pushpin selection macro to be to push the F6 key to cause "Keys" to place the selected pushpin.
There is a potential flaw to all of this and that's the question of timing. By the time you spot a problem on the road and press a key to mark the location, you will probably have travelled some way past the spot. When you combine that with the typical (in)accuracy of a consumer-grade GPS your pushpin won't be on the spot that needs the work.
Perhaps you can use "Keys" by itself to test a couple of spots while you are driving, to see if the pushpin ends up close enough to serve your purposes. If that gets it done, then you can look at using a macro utility to have different keys select different pushpins before invoking F6 to place them.
Or, since this is a commercial use of "Keys", perhaps you can commision one of the authors mentioned above to create a custom version that lets you press one of, say, half a dozen F-keys equivalent to the F6 key function, each with a pre-selected different pushpin assigned. That shouldn't break your budget, I wouldn't think.
...ken...