i80386 are 32 bits, with 32 bits of external addressing, so there's no 16 megabyte limit. The i80386SX has a 16 bit data path and 24 bit address bits, so can only access 16 megabytes.
The OrangePC 200 series have either a 386SX or a 486SLC, which is pin compatible with the 386SX. Both of these can only take up to 16 megs. Whether the four 30 pin SIMMs can be populated with 4 megs, I do not know.
The OrangePC 300 series, from what I remember, have true 32 bit i80486 CPUs which can handle more memory. None of those, AFAIK, have 30 pin SIMMs.
Commodore made the A2386 BridgeBoard with the i80386. It was intended to only take up to 8 megabytes, but a BIOS update and a SIMM adapter makes it capable of taking 16 megs. The same may be doable for the OrangePC, but I doubt it'd be easy. The notes for the A2386 might be helpful because they discuss what needed to be changed.
There aren't any PCI to ISA adapters that I know about. The ISA space and speed is too limiting to tie it to a PCI card. But you might be amazed at what kind of hardware you can get on an ISA card - new video cards, IDE / CompactFlash interfaces, USB and other things are available because of retro-enthusiasts. What kind of card do you want to add?