First, if you haven’t already, read “The origins of ràng’s svelte simplification” over at John B.’s Global Maverick.

To re-quote from Jerry Norman’s^† book Chinese (Amazon, Google Books):

As I mentioned in the comments over at GM, the last sentence interests me quite a bit. I know in 1956 and again in 1964 lists were released by Beijing giving the simplified form of a number of characters. Then a final collection (called 简化字总表) was released in 1986. What I don’t know is when 让 was introduced. My suspicion is that it was already in wide use in the Delta long before the ‘86 collection, possibly before even the ‘64.

There were of course efforts toward simplification before that. One of the earliest and maybe most relevant to the inclusion of 让 was that of linguist Qián Xuántóng^‡ 钱玄同 who published a list of around 2,400 simplified characters in 1935. The same year, the Nanjing government published their own list, though of only about 324 characters. A year before this, 中国图书馆服务社出版杜 published their own list of just over 350.

It may be of some relevance that Qián was from Huzhou, a town in northern Zhejiang where Wu is spoken.

update:
Killing time I thought I’d run through a couple dialects comparing 让 and 上 using the Wu Association online dictionary search, lacking any better resources while I couch-surf Jiangsu. Of course, they were unlikely to match since it’s neither of literary readings nor even 老派. But, at least according to their dictionary, the Hangzhou 杭州 readings do match, both being zan^҂, tone and IPA unknown. Not that I really doubted Norman’s claim. See footnote 1.

update 2:
According to an article from the Journal of Huzhou Teacher’s College published 8/99, Huzhou dialect reads 上 as [zɔ̃] or [zaŋ] and 让 as [-ã] or [-aŋ]^#. I believe the second of each pair is 老派, but I’m admittedly not at all sure. Unfortunately the chart at which I’m looking doesn’t give the initial for 让, and in fact the only intitial r- word it gives that would be either a [z] or [n]/[ɲ] is 肉 which it gives as [ɲ-], probably [ɲuəʔ]. The sample’s too small to know one way or another.

If it turns out that the second in the pair is either 老派 or the literary reading, and if 让 does in fact take the initial [z], it would support the likelihood of Qián’s influence on 让 from 讓. Of course, if neither of those are true, it wouldn’t really count as a strike against the idea.

update 3: 12/Oct
Last update, seriously. I found a rather cool book at the library today. It’s from 2007, which is æons more recent than anything else I have on 苏州话. It gives the following:

The difference between sã and sɑ̃ is minimal, so even if we ignore the literary reading, they’re still damn close.

- – -
† Who could possibly not like someone who writes papers on things like “The Origin of the Proto-Min Softened Stops”?
‡ Of no relevance here but still interesting, Qián also once suggested that Mandarin be replaced by Esperanto.
҂ If you do go wandering through their dictionary search, it may be of some use to know their transcription “ny” corresponds to ɲ/ȵ.
# I’ve run out of daggers. According to another source, 湖州新派 gives 让 the tone curve ˧˥, taking 阴去声.