Ed do you have the bucket for the front loader..
I recently saw a guy with a mini 1.5 tonne tracked digger using an inverted bucket to " capture " the post and press it in the ground then a gentle tap..tap tap etc & the post was put in the ground some 800 mm deep as easy as pie but of course the load being applied was not only the weight of the bucket and arm it was hydraulically assisted he did a couple of hundred metres of post whilst I was there.
If you can get a load of weight equal to the capacity of your machine in a front bucket why not try using the gentle raise & gravity drop of the bucket to tap a few stakes in
For the weight perhaps a water filled barrel ,sacks of wet sand , wet bricks /concrete rubble or lumps of heavy scrap cast iron if you have some , lashed in or held back by some chain link fencing .
I wouldn't go making concrete up for the job
One thing perhaps that would be useful could be a locating shoe a few inches bigger than the biggest post diameter that can be bolted securely & centrally on the bottom of the bucket so that once you start driving in a post it will not tend to slide sideways when the bucket strikes the wooden post.
Here's a nice front end knocker for a lot of work
http://www.vectorpowerdrive.com/post-driver-mod5/