I love Mark's playing - and it's interesting to follow how his tone has changed since his days with Metheny back in the 70s.
As for his sound, I'd venture that the main features of it are the Pedulla active bass with a very tough high gloss finish on the fingerboard, his roundwound strings and his technique. There are an article in Bass Player back in about 93 with Mark describing his technique - if you can find that, it's pretty insightful.
As for processing, I'm not sure what goes into his sound, he may compress it, but I don't think there's too much going on like that.
check out his website at
http://www.markegan.com and see if you can find out more there. There's an e-mail link there which may yield more accurate results.
Fretless tone is a fascnating area, with so many people choosing different variations on strings, pickups and fingerboard materials to get 'their sound' - I love the sound that players like Michael Manring, Jimmy Haslip, Mick Karn, Jaco, Steve Bailey, Pino and Alun Thompson get, but when it came to finding my own sound, I found that a less growly flatwound sound was more what I heard in my head than the bright singing 'mwah' sound that these guys get (with a great deal of variation between them, but certain characteristics that come with roundwound strings in common).
cheers
Steve
http://www.steve-lawson.co.uk
http://www.solobassnetwork.org.uk