This is not an environmentally friendly way to kill the stump. Hydrocholic acid has the following Material Safety Data Sheet:
Here is the environmental information on HCl (hydrochloric acid) from the MSDS:(just so that you don't have to dig it out yourself)
When released into the soil, this material is not expected to biodegrade. When released into the soil, this material may leach into groundwater.
This material is expected to be toxic to aquatic life.
Anyway, an alternative might be to pour vinegar (which is dilute acetic acid) in the slot. Though, I would personally just cut the stumps right to the ground and pile up some manure followed by mulched leave and then manure again. The nitrogen in the manure (don't use composted manure) will aid in the decomposition of the stump and roots.
Actually I would try to cut up what remains of the stump before adding the leaves just to increase the surface area that the manure and leaves will have contact with and to preven the tree from regrowing.
Anyway, good luck with the removal.
Anybody else have any other ideas?