Sounds like you would also have a lot of horse manure. I would make sure you were adding a good mix of straw and manure instead of just the straw. I think it would also go better if you put this mix in the spaces between all the wood and then cover the whole thing first with a layer of straw and manure and then finish with just a layer of straw. You would want to avoid any open spaces in the bed as you are building it.
You would have to be careful what plants you planted at first since there would not be any soil. But you would essentially be creating a large
compost bed with wood in it.
If you just pile up the wood and place the straw on top it is going to likely drain water really fast and dry out. This would slow the decomp a lot. Having a good straw and manure mix placed throughout and around the wood will speed up decomposition and should also retain water.
I would be very interested in hearing how it works out. Not needing any added soil would be great!