Originally Posted by David-A

Please don't be offended if I'm saying things you know, answers for benefit of others as well:

Goods being exported as checked in hold baggage, should be shown to customs before being checked in for the last time before exiting the European common customs area and VAT area. So if connecting within the area with through checked bags, you show them at the last point you check them in before leaving.

That is what the customs point is for BEFORE passenger security.
They will then stamp reclaim forms relating to thouse itmes.

Goods being carried as hand luggage should be shown to customs at the final airport you will be in before departing from the European common customs and VAT area. So if connecting, the final airport you pass through inside.

The point to do this is invariably *immediatly* after passenger security to get airside. As a connecting passenger you sometimes need to 'reverse yourself a little against the flow' / backtrack at some airports.
Customs will them stamp forms that relate to goods you show them there.

To answer your specific question, assuming you are carying the goods in your hand baggage / on your person, then YES - you can reach the airside customs point. Go up to the higher level in T5 (where people emerge from securirty into the departure lounge), and there are signs directing you to the counter so you should have no dificulty.