I don't think that the weight of the BX and mower is that significant, but if you are really concerned about the weight of the BX's front wheels running over the grass before the mower, you may consider one of the 44" or 60" tow-behind mowers such as the Swisher brand that Tractor Supply and Northern Equipment sells. These mowers can be offset so that you can simultaneously mow with your BX's mid-mower and the tow-behind (cutting up to 10' on each pass). Or, using the same offset mode you can mow grass that the tow vehicle (BX, ATV, air-conditioned truck, etc) hasn't driven over yet.
The prices run from $899 (if I remember correctly) for the 44' mower to $1699 for the top-of-the-line Honda powered 13 hp 60' mowing deck. I think that this would be significantly cheaper than a custom built configuration, and would probably have a much larger resale market (compared to the custom built front mower) if you decided you didn't like the results.
Kelvin