Oh, I'm pretty sure I understand you, don't worry. I think that the changing of time, from morning, day, and to night, is quite definitely ASM, because I'm quite sure that it is impossible to script something of the such. If it IS possible to script something that complicated, it would undoubtedly be difficult.
As for "modifying/overriding the system in G/S/C" or "replicating it in GBA", I think that it would be close to impossible to modify the system in G/S/C, although it might be possible with some ASM. ASM can make basically anything possible.
For the replication to GBA, I'm quite sure that it IS possible (as it has already been discovered by a few hackers, including Mastermind_X and ~Tutti~) using ASM. I think that it is 100% possible in R/S/E, but I'm not so sure about FR/LG, as there is no time system.
Lastly, if you want to replace certain G/S/C tiles, you don't have to import them with different palettes, as the Day/Night System in G/S/C changes the hue of the tiles (or so I'm told), so you don't have to worry about it. It would be very bothersome and difficult if that had to be done.
Oh, and don't worry, that wasn't a stupid question at all. It was actually a pretty good question. Newbies at ROM hacking usually ask really obvious question, I actually had to think to answer this one. :P