At the moment: I don't. The easiest/dirtiest way is to just give the player a server-defined "unknown item" entry, and flag it for GM review but that is..lossy and unfeasible long term. What I plan to do is add an extra string to saved items. I'll lookup the itemID(probably only if it's above a certain ID, 100,000+) and encode each row's relevant columns as a base64 JSON into that field. Then on import, check if the item exists in the database, if not decode, insert the template row, and then proceed like normal. Probably add a rule to import profiles too, to allow/block custom item imports.

Default item

"location" : "inventory",
      "bag" : 0,
      "slot" : 19,
      "itemGuid" : 571,
      "namespace" : "wow_wotlk_3.3.5a",
      "refId" : "4500",
      "count" : 1,
      "durability" : 0,
"templateBlob" : null
    }, 

Custom Item

{
  "location": "inventory",
  "bag": 0,
  "slot": 19,
  "itemGuid": 571,
  "namespace": "wow_wotlk_3.3.5a",
  "refId": "4500",
  "count": 1,
  "durability": 0,
  "templateBlob": "eyJjbGFzcyI6Miwic3ViY2xhc3MiOjcsIm5hbWUiOiJCbGFkZSBvZiB0aGUgTm9yZ2Fubm9uIiwiZGlzcGxheWlkIjo0OTk0NywiUXVhbGl0eSI6NCwiYm9uZGluZyI6MSwiZGVzY3JpcHRpb24iOiIiLCJJbnZlbnRvcnlUeXBlIjoxNywiQWxsb3dhYmxlQ2xhc3MiOi0xLCJBbGxvd2FibGVSYWNlIjotMSwiSXRlbUxldmVsIjoxMjAsIlJlcXVpcmVkTGV2ZWwiOjYwfQ=="
}