I wouldn't go down that path. The only SG verb with a present tense is 'is' so all the rest use this 'at' construction. Tha mi ag iarraidh - I am at wanting, Tha mi a' ceannach - I am at buying, Tha mi a' dol - I am at going. So while it may seem a reasonable link I think it is more of a coincidence than anything that I am wanting can mean missing. Because most of the verbs in the present tense use this construction.