Do all short sales need to first be listed on MLS?

It was recently explained to me by an Orlando FL area short sales attorney that all short sales now need to be listed on the MLS first before an offer will be accepted by the bank. That really changes how I look at the strategy of attracting non-MLS properties. In Central Florida it seems that it is most common for home owners that aren't listing on the MLS to be upside down in their mortgage. If you have to list with an agent before a bank accepts the offer, how do you effectively put in a low offer when retail buyers as well as other cash investors are likely to pay market value once it hits the MLS?

Thanks