if there was ever a case of a company making some sort of serial ASCII terminal, but in a portable factor
Well, it all depends on the definition of dumb, portable and terminal. A prime candidate would of course be the TI Silent 700 series of transportable printing terminals. Some with integrated modems and battery packs, so as portable as it could get in the 1970s.
such as a laptop or even reasonable luggable.
They have been available in any size, from luggable to fairly small.
(Image from this 1976 brochure)
While the original 1971 Silent 700, like the above 735 were quite a large beast, weighting more than 10kg, later models, like the shown 745 (below) became barely larger than an average typewriter. They were an all in one mobile solutions including an acoustic coupler or integrated modem and (optional) batteries.
(Image taken from Wikipedia)
Later models were even smaller. The 700 series was produced all the way into the mid 1980s.
While TI had a quite strong stand in the market for portable terminals, they were neither the only or even the first. For example did DEC offer their LA12 series
but I’m more curious of devices designed specifically for that purpose.
There has been quite a use case for them. Think insurance agents, mortgage brokers or even car dealers meeting their customers at home. With a terminal and a phone line they could not only show some glossy paper advertisements, but generate an offer right away.
Keep in mind, there were no portable computers in the 1970s - at least not in a way that would allow use like today's laptops. A small printing terminal could fill the gap by connecting to a central system providing the functionalit. The salesman only had to key in the customers data and not only get a tailored offer (*1), but with printout for the customer or even print the contracts right away, closing the sale in one visit.
*1 - at least that's what they were told to tell :))