so, even with a tool like yours, they can be misleaded, for
example trying a few words to replace the missing/incorrect one,
get a valid seed and stay stuck with it forever trying to play
with BIP44/49 to find their keys
Just a small detail, but my tool actually looks up all the possible combinations and then finds which one has been used before by looking for past transactions on the blockchain. Therefore, it won't tell you your phrase is correct unless it is a phrase that has actually been used before (preventing what you described).
Using some algorithm to take some input and generate a bip39 phrase that you can use with any bip39 wallet sounds perfectly reasonable.