Dateline: November 26, 2006:
The news is out of the bag. Some new replacement wiring harnesses now have plastic lamp holders for dash lights. Thanks to Pat Harris for the photo. The strange part is, these are for wedge base bulbs, so now you will have to stock another part number of light bulb for spares and keep another note for another modification to your car. And of course I hope you are not a concours enthusiast.

Original screw base incandescent bulbs were a meager 2.2 watts.
Available wedge base bulb numbers are:

Trade Design Avg. Life No. Volts Watts Amps CP Hours 161 14 2.66 0.19 1 4000 -- (very long life) 158 14 3.36 0.24 2 3000 194 14 3.78 0.27 2 2500 -- Glove box, License plate, Side marker, Instrument illumination, Door courtesy. 192 14 4.62 0.33 3 2000 -- Reading lights. 168 14 4.90 0.35 3 1500 -- (shorter life) Ash tray, Warning lights, License plate side lights.

The plastic sockets do not ground on the mounting bracket or instrument case, so they will have two wires. For the ignition warning lamp two wires is standard. The other seven dash lamps in the MGA are (were) originally all single wire with a grounding metal shell. The new replacement harness will have a black pigtail wire with a ring lug for grounding these sockets on the instrument mounting brackets, at least the remaining five instrument bulbs. I'm not sure yet about the turn signal indicator lamp, which could ground on the dash lower bracket screw.

The map light socket was originally powered by a single short red wire connected to the map lamp switch and grounding on the dash panel. A new pastic socket with the extra black wire could ground on the dash lower bracket screw. However, current report is that this socket may not be included with a new harness, and an original style screw base metal socket may be the order of the day for this spot. That's another shame, because then you need two different bulb styles for these previously similar applications.