Before I explain my question, I'll explain why I'm asking it. I use MiniBB as a forum for very old console browsers (Sega Saturn and Dreamcast). It was also used to restore the internet features to Sonic Adventure which had been offline for 15+ years.

Dreamcast has the ability to load an HTML document from it's memory card with an "x-avefront://---." command (This HTML file has to be generated with proper header data for the memory card)
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Is there a way to login by directly sending a command to index.php? I tried searching through the code of the index.php/bb_plugins.php//templates/ and could not find the command, or the correct order/combination of commands, to allow me to do that

As mentioned before, I would like to implement a memory card data generator to allow users to login with a "key" on their memory card, that would automatically send "index.php?login_commands=values". And as also mentioned, that MiniBB is used to restore personal accounts for actual in-game features, this would allow everyone the ability to have one account per memory card and login without picking up their keyboards each time they use these features. Optional "express login" button on the actual forum

If passwords in the command(s) are hashed, it's no problem they can be hashed in the generator

Thanks for all the support I've gotten so far, it's greatly helped at restoring an old game console's features

I suppose, it could be possible, but it should be a POST request formed manually. Just check parameters in the source of the login form, you should send user_usr, user_pwd and mode values, then index.php will check them in database and will set (or will not) the proper cookie. By default, it is not possible to send a GET request like you propose, this is a question of the customized script, which I suppose, is also quite possible and not difficult to program. So for example, you may code the script named login.php and organize its input and output in the way you need.

As about reading login from the memory card, I must say "sorry" because I'm not familiar with such technologies, never seen that stuff and could not give any precise advice.