There is a list of font names which are treated as system fonts even if they are sideloaded: Amasis, Avenir Next Medium, Caecilia,
Delima,
Felbridge, Georgia, Gill Sans, Kobo Nickel, Malabar,
Rockwell,
Times New Roman. (The ones in bold don't exist in the current firmware, but used to be in earlier firmware 1.9.17.)
If you sideload a font with the exact same name as one of the ones in bold, it will be treated as a system font and the advanced menu will be available. It is also possible to change the names with a patch, but I think the length of the name has to stay the same.
For example here is a patch for libnickel.so.1.0.0 that changes Rockwell to Vollkorn (note both names have 8 characters, only tested with .ttf font):
Spoiler:
Code:
<Patch>
patch_name = `Modify system font list (Rockwell)`
patch_enable = `yes`
replace_string = C35838, `R`, `V`
replace_string = C3583A, `o`, `o`
replace_string = C3583C, `c`, `l`
replace_string = C3583E, `k`, `l`
replace_string = C35840, `w`, `k`
replace_string = C35842, `e`, `o`
replace_string = C35844, `l`, `r`
replace_string = C35846, `l`, `n`
</Patch>