The problem with any direct oxidation of ascorbic acid is that DHA is actually a stronger reducing agent so this, too, will be oxidised in the process. Otherwise hydrogen peroxide would be a far less harmful candidate that iodine.
Here, for your interest, is one synthesis:
Eleftheria K. Kolioua and Panayiotis V. Ioannou, a,
aDepartment of Chemistry, University of Patras, GRC-26500 Patras, Greece
Received 29 September 2004; revised 16 November 2004; accepted 25 November 2004. Available online 18 December 2004.
Abstract
The catalytic system Cu(AcO)2–pyridine 1:4 mol % in methanol, slowly catalyses the air oxidation of ascorbic acid to the 2-methyl hemi-ketal of dehydroascorbic acid 5, and hydrogen peroxide. However, with Cu(AcO)2–pyridine 3:4 mol % the air oxidation is quite fast and no hydrogen peroxide is present at the end of the reaction. Removal of the catalyst and refluxing the foamy 5 in MeCN gives the oxidized, dimeric, dehydroascorbic acid in very good yields (70%) contaminated by 1–2% MeCN.
Of course you certainly don't want nasty stinky pyridine in your kitchen, nor do really want methanol or acetonitrile lying around there either.
But you see that the synthesis is not trivial and that is why DHA is rather expensive to buy.
So it seems to be either spend up big or just suck lots of oranges and let your body do the partial oxidation for you with its customary efficiency.