Injection of .sav files into official VC releases (both 3DS and Wii U)

I have some questions regarding both 3DS and Wii U's VC save file injection. For all the following I'm referring to in-game save files and not save-states.

For the past years I've saved most of my emulated savefiles (usually .sav). Now I want to use these .savs into the Virtual Console games, including injected unofficial ones, both on my 3Ds and Wii U. Don't know if I have to create a thread on the Wii U section for this too.

My ideal setup would be:

NES, SNES, N64, GCN, WII, WII U : On Wii U
GB, GBC, GBA, DS, 3DS: on 3DS.

I know that there are some limitations, like that most N64 games can't be injected, so for those special games I'll just emulate them.

NES, SNES, N64, GB, GBC, GBA
Those are official VC supported systems so I think injecting the .sav file can be easy enough. What's the process to do it? What's the format the VC uses?

GCN, WII
I'm aware that for GCN the savefile is read directly from the SD Card, and Wii reads it from vWii, so importing my previous savefiles should be easy.

DS
I haven't tried nds-bootstrap yet, so I don't know how the savefile works. Does it read the .sav/.dsv file directly from SD? Could I use the Drastic emu save format directly?

My idea is that since I commute a long way to work, I can play some games on the bus in my phone and then pass the savefile onto my "official" VC releases.
So any help will be apreciated.