There has been no word regarding the changing of the engagement freeze. Irrespective of the type ratings held on joining I would doubt very much if the freeze would be reduced - though this is a personal opinion. BA would have to change it's procedures and the computer software that it uses for Postings and Promotions. This would be seen as an extra cost on the business and pleasing one or two pilots would not be justification enough to change things. Similarly I doubt that BALPA would be interested in championing a change as it could be seen as disadvantaging current BA pilots.
If I were a gambling man I would take a 737 slot. The leases on these aircraft are all due to run out over the next 3-4 years and they will have to be replaced - the jury is out on what will replace them, but this would represent an opportunity to move to a different type earlier, though again this is not guaranteed. The most important thing is to take the first course offered, seniority rules your life in BA, I was lucky enough to join before Sept 11 a friend that joined 5 months later than me is approx 260 places junior. Aircraft freezes can and are waived whenever BA needs to, things change dramatically in all airlines, do not expect things to be set in stone.