3rd-party membership check

[jraderma]

My organization is preparing to upgrade from vB 4.2.5 to vB 5.6.2 and we currently check a user's membership status via a server-side GET sent to our membership services provider. In short, this uses an include that we've inserted login.php and register.php that performs the verification once the user hits the submit button. If successful, login and registration continues as usual. If not, it throws a custom error message. This was a bit clunky as anytime we upgraded where it overwrote those two files, the include would need to be reinserted.

Given the file structure of vB5 is quite a bit different from 4 and there is a lot of talk about how powerful the API structure is, I would like to ask the community here if we should scrap the clunky approach and try and build an API that will play nice with upgrades. I've never used the API option before and the documentation at http://vb5support.com/resources/api/ seems to contain only comprehensive definitions and not really any "how-to" documentation.

Thanks for any advice you may offer.
-Jon