Is it possible to create a custom detail report of customers, and using a custom field to "filter" the report.
Nope. QuickBooks does not support this.
It does however look like its supposed to do just what it says, a custom report feeding you info form quickbooks?
It will display a custom report, yes... but it won't allow you to filter it by custom fields.
Currently I am returning all the customers using a customerquery, and iterating through them one by one to get the guy I am looking for. This is becoming a slow process as the customers increase.
Have you considered instead caching the customers in your app? That would be much faster. Then you can just periodically run a query against QuickBooks to get customers that have been updated since the last time you ran the query (e.g. incremental sync of data).
I can store my own reference in an external DB, and use that to reference an email address to a customer ListID, but I would really like to achieve this with the data stored in Quickbooks only, as per the Quickbooks philosophy of, "Store Data Once"
My personal experience has been that, unfortunately, the query syntax that QuickBooks allows/uses is too restrictive for that to be realistic.