Likely relevant resources are Konrad Zuse, "Der Computer ― Mein Lebenswerk" (Springer, Berlin 1984) and Raúl Rojas (ed.), "Die Rechenmachinen Konrad Zuses" (Springer, Berlin 1998). F.L. Bauer, H. Wössner, The "Plankalkül" of Konrad Zuse: A Forerunner of Today’s Programming Languages. Communications of the ACM, Vol. 15, No. 7, July 1972, pp. 678-685 references an earlier edition of the former book as Z70 in a footnote:
From a remark in [Z70, p. 157], one can infer that Zuse already during his Berlin period, that is before 1944, used L and 0, which he called Sekundalziffern (see also [Z70, p. 68] in his diary entry of June 20, 1937).
Based on the Google Books preview of the latter book it includes the full text of Zuse's German patent application Z391 from 1941, which contains (p. 116 of the book; document 005/011 in the online Zuse archive) the following sentence:
Um Verwechslungen zu vermeiden, wird bei Sekundalzahlen die Ziffer 1 als L geschrieben.
"Sekundalzahlen" is Zuse's term for binary numbers. The sentence states that 'L' is used instead of '1' to prevent mixups. A plausible hypothesis as to the nature of the mixup in question is that typewriters in the 1930s and 1940s often lacked a dedicated '1'-key, requiring typists to use the lowercase letter 'l' instead. A perusal of a site dealing with trade in old typewriters that includes pictures of typewriters available for sale shows that this restriction also applied to at least some models of the German brands Adler, Olympia, and AEG. The use of the uppercase letter 'L' would therefore have been motivated by a desire to distinguish the binary digit from an ordinary decimal '1' and the lowercase letter 'l'.
I am not aware of any sources that suggest an origin of the 0/L notation in works preceding Zuse's, but cannot with certainty eliminate the possibility that he adapted it from some other, pre-existing work.