Andy,
Certainly not ignorance!
A valid question, and one I pondered when I restored my BM3400.
It does not have an S-meter, but 2 lamps of varying intensity to indicate when the FM signal is locked on - high-intensity when off-signal, and low-intensisty when on-signal.
On these two lamps, LED's cannot be substituted directly, without making substantial changes in the circuit, essentially transistorised drivers.
Because these lamps are NOT illuminated at high-brightness when locked on a signal, they are operating much cooler, and therefore have an extended lifetime.
So, I decided to leave only these 2 as incandescents. All others which illuminate at single-brightness levels were switched to LED's.
Conclusion (to answer your question) - it's possible, but IMHO, not practical.
Menahem
Learn from the mistakes of others - you'll not live long enough to make them all yourself!