I thought that the problem with reductive alkylation of tryptamine was cyclization to the beta-carboline, not indoline formation. Wouldn't reductive alkylation with formaldehyde or acetaldehyde likely result in the beta-carboline when using triacetoxyborohydride just as with NaBH4?
Post 68441 (https://www.thevespiary.org/talk/index.php?topic=12936.msg6844100#msg6844100)
(Lilienthal: "Re: Melatonin->5-MeO-MET", Tryptamine Chemistry)
That should not be too hard, except the unstableness of the aldehyde: https://www.thevespiary.org/rhodium/Rhodium/chemistry/indoleacetaldehyde.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/indoleacetaldehyde.html)