Principal English Translation: 

through, by, by means of, with the help of, for the sake of, thanks to (see Karttunen and Lockhart)

Frances Karttunen: 

-PAL postposition for, by means of, for the sake of / por, y mediante, como (C), por, por medio de, cerca, con, etc. (S).
Frances Karttunen, An Analytical Dictionary of Nahuatl (Norman: University of Oklahoma Press, 1992), 185.

Lockhart’s Nahuatl as Written: 

relational word. for the sake of, by means of.
James Lockhart, Nahuatl as Written: Lessons in Older Written Nahuatl, with Copious Examples and Texts (Stanford: Stanford University Press and UCLA Latin American Studies, 2001), 229.

Attestations from sources in English: 

nopal = through me
tepal = w/ someone's help

cuix no ze mopal ninemi / cuix tinechtlaecoltia / cuix mopal notlatlaqua = do you give me what I need? do I eat thanks to you? (central Mexico, sixteenth century)
Fray Bernardino de Sahagún, Primeros Memoriales, ed. Thelma D. Sullivan, et al. (Norman: University of Oklahoma Press, 1997), 297.

ma mopaltzinco xitechhualmocnoyttili = may you, by your will, have pity on us (early sixteenth century, Central Mexico)
Louise M. Burkhart, Before Guadalupe: The Virgin Mary in Early Colonial Nahuatl Literature, Institute for Mesoamerican Studies Monograph 13 (Albany: University at Albany, 2001), 118.

Ipal nonixpatlaoa. Iquac mitoa: intla aca ipiltzin itelpuch, anozo ichpuch, anozo zan itlaoapaoal, uel quioapauh, uel oquizcalti: auh ye ic yecteneoalo = Because of him my face becomes wide. This is said when someone's child--boy or girl--or else someone's pupil, was well-taught, well-brought-up, and is commended for his good up-bringing.
Thelma D. Sullivan, "Nahuatl Proverbs, Conundrums, and Metaphors, Collected by Sahagún," Estudios de Cultura Náhuatl 4 (1963), 112–113.

Attestations from sources in Spanish: 

yn occecapal ytepanco = y por otra parte a la linde de (Santa Bárbara Maxoxotlan, 1592)
Vidas y bienes olvidados: Testamentos indígenas novohispanos, vol. 1, Testamentos en castellano del siglo XVI y en náhuatl y castellano de Ocotelulco de los siglos XVI y XVII, eds. Teresa Rojas Rabiela, Elsa Leticia Rea López, y Constantino Medina Lima (Mexico: CIESAS, 1999), 222–223.

por medio de, con la ayuda de
nopal = por mi
tepal = con la ayuda de alguien
ipaltzinco = gracias a él
topal = por medio de nosotros
Rebecca Horn's notes from Nahuatl classes with James Lockhart, now in the possession of Stephanie Wood.