Usages of "sic" when citing old spelling

Yet another possible way to handle your problem is by way of introduction. In other words, instead of merely saying:

As Vanmulken wrote: "Though the fallen curry favor, the publick disregard it much."

you could say:

As the Dutch philosopher Vanmulken wrote in 1871: "Though the fallen curry favor, the publick disregard it much."

With a casual mention of the author's nationality and the time period of the quoted material, most readers will be able to figure out the reason for the "olde" spelling, without drawing special attention to it.

[sic] is only used in the case of an actual error in the quote. For example:

He said that they was [sic] calling him.

However, in your case, it seems there is no error, just a change in spelling.

I would never use it in the following example, where the quote uses a different form of English (British) from the one wrapped around it (American).

While my favorite color is red, she wrote to me 'my favourite colour is green.'

Likewise, if I were quoting Old English, I would not consider it an error within a quote if the quote was written in correct Old English.

In short, you should not use [sic] unless there is an actual error in the quote.

[sic] is reserved for mistakes as you state. Old spelling is not a mistake. You could, for example use italics for the words that differ from modern to signal to the reader that something is odd. To make such signalling clear you should tell the reader early on that this is how you have chosen to deal with the problem. Alternatively you do not signal at all and simply "warn" the reader that old spelling of words will be included.

If footnotes are allowed you could footnote the spelling the first time it occurs and provide some information on the word. This is more of a service and will not fit all formats for publishing so I mention it as an idea.

So you can come up with less obtrusive ways to deal with the old spelling. Using [sic] is as far as I can see wrong in the context of what you are doing.