hyphenate.verbatim.characters format for multiple characters?

In my PDFs (docbook-xsl v1.72.0 + fop 0.93), <replaceable> elements containing long strings do not wrap; this is particularly noticeable in narrow table columns. So I attempted to specify a number of characters for the hyphenate.verbatim.characters parameter.

My first question is - does this affect <replaceable> text?

My second question is: each format I try gives an "unexpected token" error in fop. I've tried the following formats to specify space, comma and period as breaking characters:

    <xsl:param name="hyphenate.verbatim.characters" select="',',' ','.'"></xsl:param>
    <xsl:param name="hyphenate.verbatim.characters" select=", ."></xsl:param>

Could anyone give a working example of setting hyphenate.verbatim.characters?

Thanks,

P.