I came across another usage of ところを・とこを that doesn’t match up with this grammar point at all, which makes me wonder if they should be regarded as different senses entirely.
The source is A Dictionary of Japanese Particles by Sue Kawashima (published by Kodansha). The entry is on pp. 217, 218.
ところを
Indicates that something unexpected has happened or is happening. Colloquial form: “toko-o とこを.”
English approximation: “although (usually) …”
And there are three example sentences given, which I’ll reproduce here.
主人は、いつもなら十時には帰ってくるところを、今度はまだ帰らない。
Although my husband usually comes home before ten o’clock, he hasn’t come home yet tonight.
いつもの年なら、今ごろは山は雪で白くなるところを、今年は暖かくてまだ全然雪がありません。
Although usually at this point, the mountains turn white with snow, this year, it’s warm and there is still no snow at all.
いつもなら七時には起きるとこを、今朝は目覚し時計が故障して、寝坊してしまった。
Although usually I get up at seven o’clock, the alarm clock was broken this morning and I overslept.
I noticed that this use of ところを pairs with いつも in all three examples, instead of being used in the context of an apology, or to add politeness to a request, etc. Also, since there’s a colloquial form (とこを), that would seem to imply this use is not restricted to the formal register.
Is the usage I related above a completely separate and unrelated sense? Given that A Dictionary of Advanced Japanese Grammar describes the Bunpro grammar point in the context of the 中を entry, that would seem to be the case.
