The Collaborative International Dictionary of English v.0.48:

Middlemost \Mid"dle*most`\, a. [Cf. Midmost.]
   Being in the middle, or nearest the middle; midmost.
   [1913 Webster]

WordNet (r) 3.0 (2006):

middlemost
    adj 1: being in the exact middle [syn: middlemost, midmost]

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "middlemost":
   amidships, average, axial, center, centermost, central,
   centralized, centric, centroidal, centrosymmetric, concentrated,
   core, equatorial, equidistant, geocentric, halfway, interior,
   intermediary, intermediate, key, mean, medial, median, mediocre,
   mediterranean, medium, mesial, mezzo, mid, middle, middling,
   midland, midmost, midships, midway, nuclear, omphalic, pivotal,
   umbilical