When you say "Lombard", you mean the first gen G3 that were part of the PowerBook G3 Series: Wallstreet Series I; Wallstreet Series II, aka PDQ; Bronze Keyboard, aka Lombard; and the G3 FireWire, aka Pismo - which looks like this?:
Or are you talking about another G3 Powerbook in Europe known as the "Lombard"?
The ones I mentioned above run anything from System 7.5.x to OS 9.2.2 and if you max out the RAM and upgrade the tiny 2 - 4 GB hard drive to at least 16 GB you can run OSX up to 10.4 but do not expect much power out of them. At least the Pismo with its 500MHz G3 CPU can keep up with the more modern G3/G4 machines, but the Lombard would have a bit of trouble with its slower CPU. You can get an accelerator in them to get them above 500MHz to get them to run OSX at a respectable pace however, but that is an expensive option.
As for the other G3/G4 machines, it depends on them, they can run OS9 on top of OSX but I seen some software incompatibilities with some programs, probably because of OS 9 itself or because of its emulation. I have noticed that the G3 PowerBooks and iBooks you can select to start up with either OS9 or OS Directly, and also with some G4 iBooks, but not with the G4 PowerBooks of this time (made 2003 - 2008) though they can run OS9 through OSX. The performance issue is not an issue; comparing it to the 1400cs, the G3 PowerBook/iBooks is 2X to 8X faster though there is that software issue on some programs.
It also depends on what other options you want that the G3/G4 could give, vs. what you can loose. Software issues being one of them; for me one example I can give is I noticed that Quark 3 cashes on OS9 but Quark 5 runs just fine. I/O is another, most G3/G4s do not have SCSI, so if you have scanners or external hard drives with SCSI, you lose them or need to convert them to USB, which could be expensive. Believe me I am not going to lose my 24bit Abaton Scanner that can do scans bigger than tabloid sized paper because its a SCSI!
If it were not for the price of shipping, there are a couple here who still have 1400s who are selling them in the USA. One person I deal with, J English Smith, was very helpful in having me solve a mystery to my dead 1400, which I thought was the Logic Board - it turns out to be the LCD Display. Boy do I feel dumb on that discovery, but great in now knowing what it is now to get it fixed right when I get the money and the parts. He may still have a 1400 or two and parts for it for sale, but again, shipping from the USA would be very expensive, and you would need to think if it is worth it. PM him (and others) for a price quote and then think about it.