I usually make my daily commute from East Croydon to Harrow and Wealdstone
using a zone 2-5 travelcard via Southern Trains. However, from time to time,
I find (either because I've missed a train or I have to travel at a time
earlier/later than my usual time) that I have to go through zone 1. What I
would like is a ticket that I can buy in advance and hold so that I can make
payment without having to wait at the ticket counters.

I was thinking of using carnet tickets, but apparently you can't use them
for ticket extensions into zones. I also considered Oyster, but I'm not sure
this would work either.

Does anyone have any further suggestions?

Buy your next Z2-5 TC on Oyster, and add some pre-pay credit. Then you
can travel from East Croydon to Clapham Junction, to Vauxhall, then on
the Victoria line to Euston, then on Silverlink to Wealdstone, using
Oyster all the way. The cost of a Z1 ticket extension will
automatically be deducted from your pre-pay balance.