I never even knew this existed (I try to keep a list of trouble GBA games/games with extra hardware)- others it also went by Legendz - Yomigaeru Shiren no Shima or Rejenzu in place of the first word and there was a Korean region dupe as Buhwarheaneun Siryeonyi Seom (might be worth a look the save some hassle dumping text or they might have even dropped the gimmick) not to mention I believe there was a later game in the franchise called Legendz - Sign of Necrom/Nekuromu depending upon your release list
and if you want to search レジェンズ or レジェンズ 甦る試練の島 for this game.
apparently has a copy for not a lot (about the price of a pizza) although I am not sure what it comes with.
I will have to know more about how the game and peripheral works and although it would make a great example hack if I do it as such then absolutely no dates I aim to have it done by as I have that mahjong project still on the go.
If it is a basic save device that should not be too hard but if it has more than rudimentary logic onboard then things get tricky as you are going to have to account for it (and what I could find out points towards this rather than simple saves although I might be getting confused with the standalone stuff). Your main obstacle however is that the game is not available in English.