Please consider the following 2 expressions:

for $a in outer for $b in c return d
for $a in outer for $b in c return d return e

Both are syntactically correct. In the first 'outer' is relative path and in the second it is part of "outer for" clause. To distinguish between the 2 a parser needs to look across c which is generally ExprSingle and hence - recursive. This is of course a problem for LL parsers.

Feasibility of LL parsing is not necessarily a requirement for XQuery 1.1 grammar but it is certainly a nice feature, so I wonder whether you would you consider the possibility of changing the syntax. Two options that I would suggest are:

ForClause ::=  ("for" | "outer"    ) "$" VarName TypeDeclaration? PositionalVar? 
                "in" ExprSingle ("," "$" VarName TypeDeclaration? PositionalVar? 

ForClause ::=  ("for" | "outer-for") "$" VarName TypeDeclaration? PositionalVar? 
                "in" ExprSingle ("," "$" VarName TypeDeclaration? PositionalVar? 

Thank you
Regards
Nikolay