It may be great, it may not... that's definitely an empirical question due to the many factors:
- how much of the xmit current on the ground leg will be parallel and next to your vertical? this could cause destructive interference and make the antenna pattern more directional.
- are there other currents on the ground that will make their way into your antenna system as RFI?
- prob more.
If this is primarily for 10/20m, you might first experiment with dropping a simple 1/4 wave wire attached to the coax shield at the feed point to create a place for the "ground leg" of your RF current to operate. I would expect to find an improvement in SWR as well as both xmit and receive capability.
I should add: since the goal is to provide a low impedance path for the shield current, it can really be any multiple of 1/4 wave... so if 20m is your lowest frequency, cut to that wavelength which is 2/4 waves at 10m and still good.
There are some GREAT Q&A here on the topic of grounding, I've learned a ton by perusing.
Especially relevant, this answer to a closely related question. In part, Phil notes:
"RF ground" usually means "something that is at the same potential as the soil". This is important because if you have a wire (such as your feedline, for example) which is not at ground potential, then there exists a non-zero electromagnetic field between that wire and the soil. That means the feedline is radiating/receiving, which is usually undesirable.
It's worth checking out the entire answer.
Unrelated: if you haven't already installed an RF choke on the feedline at the feed point, you should consider doing so to keep RF off your coax and keep it going thru your antenna. Likewise, you should consider another RF choke on the feed line at the receiver to keep any environmental RFI that enters your feed line out of your receiver. I've found this to be very helpful in some of the dense living situations I've encountered :-)