finally i've done it, i rewrite here my experience hoping will be useful to someone.
source materials:
http://www.mediafire.com/file/x8anxwhp6umauzn/usbloader+gx+r1262+plugin+mod+v2.rar
http://www.mediafire.com/file/3mxh2azkbza43db/WiiFlow_v4.2.3_Nintendont_MasterMod_Pack_Full_2016.7z
https://gbatemp.net/threads/cover-collections-for-emulators-with-cover-support.324714/
i suppose using sd card, already having usbloader_gx for wii & ngc, wanting to emulate nes.
first update the usbloader_gx with the files from the first archive. they go in:
sd:\apps\usbloader_gx\.
then make a folder "plugins", so you have sd:\apps\usbloader_gx\plugins\.
navigate the mastermod pack in \wiiflow\plugins and copy fceugx.ini and
fceugx-mod12.dol to your sd:\apps\usbloader_gx\plugins\.
open fceugx.ini in a text editor, you'll have:
[PLUGIN]
displayname=FCEUltraGX
magic=46434555
coverfolder=fceugx
dolfile=fceugx-mod12.dol
romdir=fceugx/roms
filetypes=.nes|.fds|.nsf|.unf|.nez|.unif|.zip|.7z
consolecoverid=nintendo
covercolor=ff0000
bannersound=nes.ogg
arguments={device}:/{path}|{name}|{loader}
you can edit to this:
[PLUGIN]
displayname= Nintendo Entertainment System
magic=46434555
coverfolder=covers-nes
dolfile=fceugx-mod12.dol
romdir=games-nes
filetypes=.nes|.fds|.nsf|.unf|.nez|.unif|.zip|.7z
arguments={device}:/{path}|{name}|{loader}
make these three folders:
sd:\games-nes\
sd:\apps\usbloader_gx\images\covers-nes\
sd:\apps\usbloader_gx\images\2d\covers-nes\
put your games in sd:\games-nes\, now you'll have:
sd:\games-nes\game1.zip
find the right covers in gbatemp thread indicated and put your 3d cover in
sd:\apps\usbloader_gx\images\covers-nes\
and your 2d cover in
sd:\apps\usbloader_gx\images\2d\covers-nes\. now you'll have:
sd:\apps\usbloader_gx\images\cover-nes\game1.png and
sd:\apps\usbloader_gx\images\2d\cover-nes\game1.png ;
rename both game1.png to game1.zip.png.
open usbloader_gx and check these settings:
"plugin covers 3d" ->on/off on "gui settings";
"plugin default partion" ->sd on "loader settings";
"plugin file path" ->sd:\apps\usbloader_gx\plugins on "custom paths".
if you've done right these lines will be in your gxglobal.cfg file:
PluginsPath = sd:/apps/usbloader_gx/plugins/
pluginMode = 1
enabledPlugin = 46434555
Plugin3DCvrs = 1
PluginRomPart = 0
choose the plug icon in the main view and you'll see your games!
you can use sd:\config\plugins\ instead of sd:\apps\usbloader_gx\plugins\
to avoid homebrew launcher shows you other dol files inside usbloader_gx folder.
a big thank to all the people who worked on this fantastic software,
hoping they will continue to develop and improve it!