Qui Episcopum ad civile tribunal vocat, ipso facto excommunicatione speciali modo Sedi Apostolicae reservata plectitur.
Canon 2341 assigns a single penalty, excommunicatione speciali modo Sedi Apostolicae reservata, for summoning any bishop before a civil court, and unlike Canons 2331 or 2343 it contains no hierarchical distinctions whatsoever, making your claim of a “lesser reservation” simply incompatible with the text.
Slap this in Google Translate (it's getting pretty good now).
2341 Si quis contra praescriptum can. 120 ausus fuerit ad iudicem laicuм trahere aliquem ex S. R. E. Cardinalibus vel Legatis Sedis Apostolicae, vel Officialibus maioribus Romanae Curiae ob negotia ad eorum munus pertinentia, vel Ordinarium proprium, contrahit ipso facto excommunicationem Sedi Apostolicae speciali modo reservatam; si alium Episcopum etiam mere titularem, vel Abbatem aut Praelatum nullius, vel aliquem ex supremis religionum iuris pontificii Superioribus, excommunicationem latae sententiae Sedi Apostolicae simpliciter reservatam; demum si, non obtenta ab Ordinario loci licentia, aliam personam privilegio fori fruentem, clericus quidem incurrit ipso facto in suspensionem ab officio reservatam Ordinario, laicus autem congruis poenis pro gravitate culpae a proprio Ordinario puniatur.Special Mode Reservation -- Cardinals, Legates of the Holy See, Major Officials of the Roman Curia. his own Ordinary
Simple Mode Reservation -- Any Other Bishop, even merely Titular, an Abbot or Prelate in charge of no one
... Suspension from Office and other TBD by his Ordinary -- anyone else who's subject to Canon Law
So it says here that you get a simpliciter reservatam (to the Holy See) excommunication for bringing anyone to the court, even if they're a "Prelate of no one" (which seems to suggest even if they have no ordinary jurisdicition or MIGHT be synonymous with being at least a "merely titular" bishops, not sure).
Where there might be some wiggle room is whether someone must be a bishops with some kind of rank, at least Titular ... since +Lefebvre had resigned his positions before the Nine brought their suit. I believe that they would certainly have incurred the last part, suspension from office etc. ... except of course, Traditional priests don't really have office and are in a quasi-suspended state anyway.