How do you unlock hidden songs in older Guitar Freaks & Drummania games?

I recently asked this in Reddit but I didn't get a definitive answer so I thought to ask here.

I was recently playing the older Guitar Freaks and Drummania releases in MAME and noticed some songs are missing, for example, Hypnotheque and Pretty G in Guitar Freaks 6thMix and Drummania 5thMix.

Searching the site VJ Army (now RemyWiki), I found out they are hidden songs. However they never mention how to unlock them. Those games don't have e-amusement like current releases so, how do you unlock those songs?

Thanks.

EDIT: someone answered the question in Reddit.
In case anyone wants to know how to unlock these songs, follow this link:
https://www.reddit.com/r/bemani/comments/cfvqtr/how_do_you_unlock_hidden_songs_in_older_guitar/

It is a (very) slow process, but we're slowly trying to fill in song unlock info for older BEMANI games on the wiki. Just might take a while, since a lot of info is exclusive to Japanese websites.