Sentence Correction

Can you have "which" after a hyphen? or it can only come after a comma?

example:

The store has organized a campaign--which used websites, public meetings, and etc...

Many times a dash can be used in place of comma but it is a mark of more stronger separation.

Reference The Elements of Style:
Use a dash to set off an abrupt break or interruption and to announce a long appositive or summary.
Use a dash only when a more common mark of punctuation seems inadequate.
His first thought on getting out of bed -- if he had any thought at all - was to get back inagain.begins with which but not sure of summary here.
An appostive never
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Reference dictionary.com
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the mark or sign (-) used to note an abrupt break or pause in a sentence or hesitation in an utterance, to begin and end a parenthetic word, phrase, or clause, to indicate the omission of letters or words, to divide a line, to substitute for certain uses of the colon, and to separate any of various elements of a sentence or series of sentences, as a question from its answer
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Hyphen: a short line (-) used to connect the parts of a compound word or the parts of a word divided for any purpose.

I can not think of a situation in which I can add which after -- but it looks concievable. I let experts comment to give an authentic answer.