APY,
“walking hand in hand into the seuda would certainly qualify as an act that demonstrates they are husband and wife”
See the below link:
http://www.hebrewbooks.org/pdfpager.aspx?req=46545&st=&pgnum=219
Footnote Alef
However, he does claim that after the Yichud it has no source.
See end of footnote Vav/6:
http://www.hebrewbooks.org/pdfpager.aspx?req=46545&st=&pgnum=230
Well, I guess people apply the ballroom as the “Bais Nesuan”…