You perhaps know this, but the planning process is not linked to the existence of a covenant on the land. Only the holder of the covenant can take action on this - so find out how it is worded (whether in favour of an named individual, or whether tied in perpetuity to the owner of another piece of land, or some other way). You then may be able to find out whether there is likely to be anyone able to take action. And even then it probably won't work because the individual in whose favour the covenent works will probably merely want to extract some money from the developer in exchange for a removal of the covenant.
In reality, there probably isn't anything you can do and the developer knows it. His only issue is going to be how to sell the houses, once developed - because the covenant will of course still apply. Since indemnity insurance can be purchased to guard against the costs incurred if the covenant-holder turned up later, he will probably solve the problem that way.
You best way is to gavanise effort to find a planning reason why the land should not be developed - not rely on a covenant.