dear Juan Carlos,
my first impression is the same that you have had, namely that a local outflow tract thrombosis, although recanalized, may have caused a regional pre-hepatic portal hypertension.
However, I wonder whether ultrasonography with Doppler can detect some of the colonic varices or collaterals to test whether the flow is flat and venous rather than phasic / arterialized. The latter instance may suggest an arteriovenous fistula, causing local portal hypertension.
Would it be possible to consider arteriography to evaluate separately the vascular beds deriving for the inferior mesenteric artery and superior mesenteric artery, with venous phases too, to confirm whether signs of long standing venous thrombosis or rather arterio-venous fistula exist?