Found some information that might be relevant here:
MSDS for Picklex [PDF]
Less than 10% phosphoric acid
MSDS for Rust Mort
(Leads to a PDF) phosphoric acid 25 - 50%
I'm kinda partial to 19th century writing, so I dug up an old book:
A Dictionary of Chemistry, On the Basis of Mr. Nicholson's; The Principles of the Science are Investigated Anew, and Its Applications to the Phenomena of Nature, Medicine, Mineralogy, Agricultures, and Manufactures, Detailed.
By Andrew Ure, M.D., 1821
"Diluted muriatic acid rapidly dissolves iron at the same time that a large quantity of hydrogen is disengaged, and the mixture becomes hot."
"Phosphoric acid unites with iron, but very slowly."
I don't think basic inorganic chemistry has changed all that much since 1821, so you have a choice of acid that dissolves or acid that unites.
BTW, keep in mind that one of the things produced from putting muriatic acid on iron is ferrous chloride -- used for etching -- so it's an accelerating process. Leave it on too long (how long is that?) and you could end up with holes or hydrogen embrittlement. Leave phosphoric acid on too long and you get iron phosphate -- a rust inhibitor.