Is there any merit in adding a tweeter and simple crossover (i.e., capacitor) to suppliment the 15-inch speaker on the Seeburg M100C jukebox? Although I am not sure where I could mount it...

Am I asking too much of that sound system? I would like to do more than just boost up the treble on the amp, even though it might bring out the worst in the old 45s.

Yes, see the seeburg hf-100g hifi with a little metal 5 inch tweeter...and a .5 mf as
crossover. The amp is slightly different as well..it is the last to use field coil main speaker.
The issue is really the beautiful little red head cart that looks so cool..or if you are still using the original issued black head...either of these is your limiting factor...the overpriced pickering replavcement is nice...the "thorpe grey" aftermarket carts on ebay are good..The red head or black head are ok for pre 1959 or so discs after that some good , more bad..
I tried to add just a tweeter to a seeburg, and though process of elimination , discovered the amp was capable of the sound I wanted but it needed a better source, namely stylus size, pick sensitivity and riaa curve for better brighter sound that the new vinyl discs are capable....here is a sample (my father in law's band) on my G with thorpe cart.Listen to highs stock system newer cart
http://www.youtube.com/watch?v=405_8JxPT80

I can't say I've ever been much of a fan of the Thorpe cartridges. They're low cost ceramics stacked back to back. To me they aren't as clean sounding and don't track as well as the Pickering. I also wonder if their high output voltage doesn't overload the Seeburg preamp. The 345-03D Pickering are expensive now, because they're discontinued. But Vern Tisdale has developed a nifty solution that's about half the price.
http://www.verntisdale.com/cartridge.htm
As for adding a tweeter to the C amp, I see no reason why not.
I'd suggest a large cone type unit.. something you can cross in at a low enough frequency. Maybe you can make an arrangement where you could mount a tweeter in the middle of the woofer cone.

Sounds familiar.
Many of these Jukes were upgraded to the factory redhead cartridge at some point in their life without paying any attention to the amp. If this is the case your sound will suffer.
If in fact you have the original amplifier installed and a red Pickering cartridge please make sure the input resistor has been changed from 10K to 47K. (going from memory)
If the cartridge is the original black Pickering unit it can't hurt to verify the input resistor is correct.
Vern is no longer producing the replacement cartridge. Health reasons.
If you are using the redhead they work much better with the aftermarket dogleg needles and not the original straight nail type. Unfortunately the newest needles are poor and result in decreased volume. Best to sub another cartridge for test purposes. Also have a good look at the speaker. After all that and if the amp is good/rebuilt then adding a tweeter is perhaps in order.
Hope this helps,
Mauro

The Tisdale Cartridges sound great to me (wish I had more) but unfortunately they are no longer available to the best of my knowledge. Seeburg cartridges and needles are getting tough to get; hope a solution appears but?
DAvid (Geritol Ghetto)