Custom fields validation - server side

[asherxo]

I've succesfully created a few shopper field ("custom" field type) in order to collect another user data.
I need to validate this field according to some specific walmart one login rules: when using "register and checkout" mode, i've seen that's "plgVmOnBeforeUserfieldDataSave" method I could use for this purpose. What if using "Checkout as guest"? I didn't found a way to intercept a plugin event in order to validate this field. Any suggestion?
Thanks in advance