I did something similiar a few years ago with additional demographic information but not necessarily portal info. We used the additional information fields during registration to store the data. As you add new fields to be populated during registration you can pull their importID from SQL in select * from hie_pm.IB_patient_addl_info.
Then you need to update the appropriate xsl to send the values in the ADT message. If you were trying to send the field with an importID of 6 in ZAL03 it would look something like this.
<xsl:value-of select="PAT/Field6/text()" />
As far as inbound to EHR I know there are some Portal related parameters in the 253-266 range but I personally haven't dug into them before.
So, I don't think there is really a standard out there now. Are you needing to import the data into PM or are users manually entering it at time of registration?
Galen Healthcare Solutions