Hau Matt, welcome to permies.
Sounds like a good idea to me. Those bales, if not spread out before putting the cover on, will hold a lot of
water for at least five years of use.
I have some
straw bales that get planted in, we use them for a period of two years then plop new bales on top of the decomposed bales.
The best I've seen for lasting time was three years, then the bales were so decomposed they became the base for new bales.
When you are starting new bales, you need to get them started decomposing (unlike a
wood filled growing mound, these have to be encouraged to be decomposing before you plant anything in them).
We set our bales out and then sprinkle used
coffee grounds on the tops, water it in and repeat this process every day for two weeks. This gets nitrogen into the straw and heats up the interior, starting the decomposition of the bales.
Next we sprinkle dirt on the tops and water that in, we repeat this process every few days for two weeks.
After all that, we are ready to plant in the first year bales. You stab a hole in the top of the bale and plant the seeds in the stab holes, fill them with soil and water.
Anything except for root crops (carrots, potatoes etc. can be grown in bales treated this way.
Potatoes do great if you start the bales as above but before you plant your potatoes you have to cut the strings that hold the bales together and remove at least half of the bale (which you then use to cover the growing potato vine to make the "Hill".
With the round bales being used as the logs, you really wouldn't need to water in any nitrogen or soil before you cover them.
I would however wet them down until water leaks out all the way around each bale before I put the dirt cover on.
If you aren't fully covering the bales with dirt, you are making bales as I described above, not a mound.
By the way, strawberries love to grow in the sides of straw bales that have been given the treatment I described above.
