You and the OED are apparently plowing the same ground, David.
They issued a draft entry on rancio in December 2008 and apparently conclude (if I'm reading correctly) that "rancio" has a different derivation. I'm impessed, frankly, with the depth of analysis of the word -- in the past, some technical wine words have been pretty superficial, even misleading -- I remember Robin forming a similar judgment some time ago.
In any event, here's their current take on "rancio":
[< French rancio wine having a distinctive rich flavour or bouquet, redolent of nuts, butter, or overripe fruit, produced by oxidation or maderization (1755; a1755 as adjective in vin rancio, designating such a wine), characteristic flavour of such a wine, quality of possessing these characteristics (1869) and its etymon Spanish rancio (noun) characteristic flavour of an overripe wine, quality of possessing these characteristics (a1606), wine having a distinctive rich flavour or bouquet, redolent of nuts, butter, or overripe fruit, produced by oxidation or maderization (although this is apparently first attested later: 1809 or earlier), (adjective) (of wine) becoming more delicious and mellow with age (1653), spec. sense developments of rancio (adjective) rancid (1490), (noun) rancidity (1495) < classical Latin rancidus RANCID adj.]
A type of (esp. French or Spanish) wine having a distinctive rich flavour or bouquet, redolent of nuts, butter, or overripe fruit, produced by oxidation or maderization; wine of this style. Also: the quality of possessing these characteristics, associated with certain strong, fortified, or dessert wines, or cognac. Freq. attrib.
Used in France as a legally defined designation for certain white and red fortified dessert wines.
1800 tr. L. S. Mercier New Picture Paris II. cliii. 91 Throats, which had hitherto swallowed nothing but water, poured down the wines of Beune, of Nuits.., of Paquaret, of Rancio [Fr. de Rancio], Cape and Hungary wines. 1859 Times 15 Feb. 9/5 [The wine of Tallano] will bear comparison with the Rancio of the Pyrénées. 1879 Franco-Amer. Commerce (San Francisco Chamber of Commerce) 38 The flavor and bouquet of genuine cognac is produced [in imitation cognacs] in several ways. One is to add..a little Jamaica rum to give what is called the rancio. 1901 Atlanta Constit. 12 May 36/7 The fee of the cure was 6 francs 50 centimes and a bottle of Rancio wine. 1939 B. COLLIER Catalan France viii. 200 With dessert we drank an excellent Rancio from the neighborhood of Barcelona. 1988 Gourmet Oct. 211/1 The air space..allows flor..to bring to Tokay a unique combination of intensity and delicacy, sweetness and nutty rancio. 2001 O. CLARKE Encycl. Grapes 97 They acquire a maderised, sour rancio tang of nuts and raisins and cheese. 2005 Albuquerque (New Mexico) Jrnl. (Nexis) 30 June S1 The new-style Priorat Garnachas are far better wines, if a bit less interesting, than the oxidized rancios of yore.