Today I collected the 1954 English edition of Jeremias’ The Parables of Jesus from the library and discovered that the information about the Aramaic le is there on p 78. I also read a couple of the footnotes with new eyes and realised that although they said “Examples: …” they might not be simply citing editions of Aramaic texts, but older commentators. Ooops! And yes, kind of: some of the examples are in H. L Strack & P. Billerbeck’s Kommentar zum N. T. aus Talmud und Midrasch vol II (München, Beck, 1924) pp 7ff and others in Paul Fiebig’s Rabbinische Gleichnisse (Leipzig, 1929) – many pages.
Strack and Billerbeck is available on-line at https://archive.org/stream/KommentarZumNeuenTestamentAusTalmudUndMidraschVol.2/Kommentar.Strack.Billerbeck.v.2#page/n23/mode/2up but it seems to me that they are not taking the same line as Jeremias. They simply say that the Aramaic le is the equivalent of the German ‘gleich’ (like, similar, the same as), rather than pushing it as far as Jeremias does ie suggesting that it pushes the focus away from the direct object to some other part of the sentence.
Unfortunately, none of the libraries to which I have access have a copy of Fiebig, and it probably isn’t important enough to chase it by interlibrary loan. WorldCat suggests that there are no copies in Australian libraries, and although this, as I established earlier, is not always reliable, it is certainly not important enough to get on international ILL. I think I will need to conclude that Montefiore is probably guilty of sloppy referencing because he is likely to have picked up his comment about the Aramaic le from one or more of these three authors and that Jeremias has pushed his translation theory further than at least Strack and Billerbeck did in their treatment of this construction.