Some run a wire along the ground under the h portion of the L for its entire length, in addition to the radials at the v portion.
This is exactly what I do, with good signal reports.
And no reason why not.
The purpose of the ground radials is to act like like a mirror of the radiating element which is the vertical section and is why concentrating the radials there is best.
In my own present setup, I can't run a radial the full length of the horizontal anyway as it runs over a neighbours field. I do have about 30 unused radials at the end of the horizontal wire from a previous setup which I may connect to the current setup. The more the better!
I'm lucky that I have a proper Field Intensity Meter (Nems Clarke WX-2E, similar to a Potomac) so any changes I make can be accurately measured.