I don't have the original books anymore, so I can't say one way or another about it.
I still feel it comes down to economics. The difference between what you say and what I've proposed is that each government holds an effective monopoly for its production within each country, which it may trade or not as it see fit. All that will do is artificially skew the demand/supply ratio for any tantalum that is traded on any sort of market, free or closed. For tantalum that each country holds internally, the government still has to allocate whatever monies it feels the extraction and processing of the material is worth. Just because each government controls its own supply of tantalum doesn't mean it got it for free.
Besides, if ALL of the supply is in government hands, what does that say about any commercial ships that require it? Does that mean all ships are directly backed by the government? Or do governments allot a specific amount for commercial use with the price being the high bid ( in which case at least a portion of the tantalum market is demand/supply constrained ). Or maybe it goes to whatever corporation that the current government likes for the moment ( oooooo.... political intrigue and lobbyist scandals! ). Hmmmm...multi-national corporations playing different governements against each other for best prices...bwahahahahahaha!!!.... after all, its the government paying these same corps to extract it for them, right?
Mussolini said:
“Fascism should more appropriately be called Corporatism because it is a merger of state and corporate power”
Perhaps there are overlapping spheres of influences between govs and corps that are separate from and ignore national borders....more lolz
Apparently, tantalum in 2300AD is handled the way weapons grade plutonium is handled by governments today. So, perhaps its not that raw tantalum is so strictly controlled as much as the processing of it is... complete with multi-national oversight?... still costs the governments money and thus economics is a limiting factor.
http://www.globalsecurity.org/military/library/report/1988/JGA.htm
This paper would seem to indicate that the control of such strategic materials has a large influence on diplomatic and economic policy.... which opens up quite a few scenarios for exciting adventures, eh?
Moving to acquire strategic materials or the actions to deny them to a cold-war enemy might involve a few 'police actions' and proxy wars for those who like to play with a military slant
The shenanigans that corporations might cause in order to gain the government allotted commercial amounts might be fun to use an adventure hooks too.
well..thats my opinion anyways