Without having gone into the subject in any depth, the impression I get from the USSBS is that there was ample scope for employing the ore carriers in the role for which they were designed.
Disregarding their possible use as colliers plying between Rashin and Hokkaido/North Honshu ports and/or on Yellow Sea - Japan routes, two significant employments come to mind:
1) Low grade manganese ore shipments from the Philippines following loss of India as a source of high grade manganese;
2) Dramatically increased shipments of bauxite, notably from Bintan Island, just south of Singapore.
The 81 resource centres you see in the game at Bintan doubtless represent this important source of bauxite to the Japanese. They had 250,000 tons of bauxite stockpiled at the war's start - enough for just 7 months production - and pre-war embargoes had depressed annual bauxite imports to 147,000 tons in 1941. According to USSBS, this increased to 450,000 tons in 1942 (no doubt aided by Bintan's capture), and to a peak of 820,000 tons in 1943 before the US submarine offensive began to bite in 1944.
I don't have anything in the way of original source material that would assist, but am aware that there are several posters on
the j-arcraft Japanese Ships and Navy board who seem to have access to extensive data on Japanese convoy movements, and the ships making up such convoys. A query raised on that board about movements of Standard Type 1K ore carriers might well produce helpful information.