This has all been discussed in the past. Currently we do not allow for "modified" or "generic" entries outside of some cables that end up being in Accessory categories.
The reason that we can allow hoshichiri's custom GBA is because it is a complete product from a business. In addition, we can also allow each of the individual custom cases to be added as accessories (the ones I linked to referring to the colors) as they are separate products.
Region modded consoles would not get any additionals. You select the console that is closest to what you have, or pick whatever one to represent the thing you own, and put into notes that it is modded or what have you.
A chipped console would not get their own entry, although an entry for the chip itself is allowed to have an entry in the appropriate accessory category. Same would go for any consoles that get region or AV mods by way of add-on boards that get soldered directly to the PCB. Those items can get their own entry because they are products purchased from somewhere.
Homemade modifications are not allowed separate entries, but the individual parts are.
If any of the people had bought that adapter you made, then they could add it if they wanted, but there is no requirement that it needs to be added.
We (at VGC) can potentially do more, but not within our current design. It is possible to accomodate those with custom consoles without adding generic entries, but at this time we do not have support for that.