What a laptop personal computer is and precedence of Toshiba T-1100
I have no doubt about the relevance of Toshiba T-1100 in the development of laptop personal computers. It could be, in fact, the most successful out of all of those early laptop personal computers. So, such a citation as you are suggesting can be very appropriate to my point of view.
I think, however, that the noun “pioneer” can only be used to refer a person, being more correct to use the adjective “pioneering” in order to mention new ways of doing things, which others later follow. For example, “Pioneering work in the field of…” or “Pioneering contribution to the development of...”.
What about the citation: “Toshiba T-1100, a pioneering contribution to the development of laptop personal computers”, to name your milestone? Of course, needless to say the exact wording is subject to final discussion of the IEEE History Committee.