Interesting problem again. You're making headway through Aquinas! Even in Mediaeval Latin, the commonest words for a despot were ''tyrannus'' for a despot or ''dominatio, -ionis'' or ''tyrannis, -idis'' for the abstract noun (despotism).
One of the problems I often find with Aquinas' scholastic Latin is that it can significantly depart from the vocabulary of Classical Latin because of the demands of its use as a vehicle for largely new concepts that the original language did not possess.
There is a vademecum based on the dictionary of Saxo Grammaticus that was updated to 1997 and is available online, if you don't already know of it: http://www.rostra.dk/latin/
Sadly, I don't know of the word ''dispoticus'' either and I have checked my complete Lewis and Short and the Oxford Dictionary of Mediaeval Latin as well!
I would certainly support your interpretation of the word as being ''despot'', which has a more perjorative meaning than ''regale'', which would convey the notion of appropriate rule and majesty. In Aquinas' thinking, a despot would be an illegitimate ruler, while one that was ''regale'' would be seen as having the qualities of legitimate and ordained rule. I think it's the connotative meanings of these two words that make the difference rather than their purely denotative meanings.
If you look at the link in this passage between ''anima'' and ''dispoticus'' versus ''regale'', Aquinas' fundamental point is that the body is at the complete and utter control of the soul: it has no choice but to obey whatever commands it makes. (Remember that ''anima'' can also mean the mind.) In other words, in this instance his argument by analogy is that the soul/mind has the same kind of total control over the body that a despot has over his people. It is not kingly because even a subject has the right to refuse a knowingly wrong command issued by a monarch and a monarch cannot (theologically at least) enforce obedience when it will place his subjects in peril of sin. It's a complicated analogical argument that is shaded by all sorts of theological arguments about the legitimacy of rule that Aquinas' contemporaries would have implicitly understood, which is why it doesn't make immediate sense to a contemporary reader.
I hope this helps and sorry it's so long winded!