I think that if we can prove that some the Kulos in Besamim Rosh mirror Rabbi Berlin's Reformist agenda as can be seen in his earlier writings that might be a partial proof of forgery. Here is one example:
In his K'tav Yosher ( a fascinating work of its own right) he sarcastically has one of the characters announce (pg. 8):
לו יאבה מלך לתת לי עד חצי המלכות ואשתה מחלב שחלבו גוי לא אעשנו
showing that he places this law on the same basis as, e.g.:
"Did he [Wessely], God forfend, trample upon such good and pleasant customs as swinging a chicken around one's head on the eve of the Day of Atonement striking the floor with a stick at the mention of Haman on Purim, or eating the head of a sheep, cabbage, dates and green vegetables on the New Year?"
and in Besamim Rosh (no. 36) he does cite arguments that would undermine the basis for the law of Chalav Akum.
[NB: But note that Chcham Yosef Messas writes in his Mayim Chaim V. 2 that in Morocco they had an over 300 year old custom to be lenient on Chalav Akum for reasons much like those of R Berlin]
[A copy of R S Berlin's Mitzpeh Yekusiel can be found both at hebrewbooks and JNUL]