Simple patch including the startup menu & replacing Guide with the map debug menu:
(patch also includes essential fixes and 6C destruction fix because why not)
(EA) Source:
#include "Extensions/Hack Installation.txt"
ORG 0x000AD8 // ORG somewhere in the startup routine (where the call to the GAMECTRL constructor was)
BL(0x01C090) // Call the debug startup menu
ORG 0x59CEC0 // ORG to guide routines
POIN (0x04F448+1) // Always usable
WORD 0 // No Fancy draw
POIN (0x01BB1C+1) // Debug Map Menu
The bootup menu seems to be fully functional. the map menu however seem to have quite a few missing features (in fact it will crash no$ because it tries to reference something that doesn’t exist anymore (see notes below)).
I found one smaller menu too (the “Charge” menu, located at 0859CFD4, routine to open it at 0801C2E4), but again it doesn’t seem to work properly.
I spent like 4 hours looking for something that could look like one of those nice character menus without much success… So I guess we still don’t know if it isn’t gone or not :/.
Credits to lin for hooking me up with material on the prototype debug functions.
Debug Functionnalities Restoration Patch 
??? (probably not but maybe)
Hopefully those details will work
FE8U DEBUG REMNANTS NOTES
Routines related to debug seem to be located around 0x01C000
0801C090 is the Debug Startup routine, the following EA code will replace the standard one with it:
ORG 0x000AD8
BL(0x01C090)
Seems to work 100% well
0859D040: Map Debug Menu Def
0801BB1C calls that, but seems to also be a menu effect routine (takes Menu 6C as r0, returns 1 etc)
08033468 calls that, but seems to be for prep screen?
This replaces the guide menu command with the map debug menu:
ORG 0x59CEC0
POIN (0x04F448+1) // always usable
WORD 0 // no fancy drawing
POIN (0x01BB1C+1) // calls the map debug menu
will not work in no$ because of the following:
6C at 0859AA5C (blank, deletes itself immediately) seems to be looked for by a bunch of debug routines, so I think it's safe to assume this used to be some "debug manager", but now is removed.
Known 6C 0859AA5C Fields:
58 | word | weather id (0 => 0, 1 => 6, 2 => 1, 3 => 2, 4 => 4, 5 => 3, 6 => 5)
imma slep now 