Yes there is number of after-market alloy pans offered by after-market parts outlets for the T400 - I considered purchasing one but decided it was not appropriate for the Corniche when repairing the transmission as below.I think the idea behind no drain plug was to motivate you to replace the filter which is important and cleaning the magnet that is usually found within the pan. Notice the grey sludge...that's from the multitude of gears, plates, bands, etc. The drain plug makes draining so much easier and mess-free. I have seen cast pans with drains for TH400's.
The T400 on DRH14434  did not have a magnet in the oil pan as installed by RRMC however I did add one when reassembling the transmission after a major fluid link when the pipe to the transmission oil cooler decided to be incontinent and the transmission clutch packs failed when I managed to force the car up my drive into my garage with the transmission haemorrhaging fluid everywhere. so it could be repaired with a B&M T400 overhaul kit I sourced from the USA.
T400 oil pans - Summit Racing