There is an obscure way to get invalid D05E values without ACE, and that is with a glitch I found in 2015 known as participants glitch (which I believe Crystal_ helped me with). It requires over 6 Pokémon, which can be achieved at the beginning of the game by using the SRAM glitch to get 255 Pokémon.
Basically D058 in Red/Blue or D057 in Yellow is the 'participants' address. Bit 0 of this address means Pokémon 1 participated and will gain experience, bit 1 means Pokémon 2 participated, and so on up to (unused bit 0x7) Pokémon 8. However, the game won't stop you from manipulating later addresses by sending out Pokémon beyond slot 8. This has some fun uses, such as forcing a Pokémon battle (Mew and MissingNo. possible but not much use as if you have expanded party you can usually get expanded items pack) via writing to D059 after sending out specific Pokémon (the battle can then be forced with Red/Blue's 9F (hex:5E), Yellow's -gm (hex:6A) and Lg- (hex:6E) and forcing a different D05A (D059 in Yellow) battle mode such as old man/Safari Zone/Professor Oak/invalid battle modes (unfortunately the invalid battle modes seem to be all the same).
Apparently D05E is in range with participants glitch. If D059 is written to by Pokémon 1-8, D05A is written to by Pokémon 9-16, D05B is written to by Pokémon 17-24, D05C is written to by Pokémon 25-32, D05D is written to by Pokémon 33-40 and D05E is written to by Pokémon 41-48.
So the theory goes like this;
1. Document the invalid hitflags text box sources with BGB and see if they execute arbitrary code, or the source text pointer can be forced to do that with an 08 command. Then hope and pray there is a reasonable execution pointer (probably the only convenient ones here would be in items or stored Pokémon).
2. Convert the ID to binary, then determine which Pokémon (from slot 41-48) need to be sent in battle to write the given D05E value.
3. See if you can activate the glitch text box after making a move. (I actually haven't tried this yet, it may be these values need to be locked to D05E, which would be a shame.