If you need a larger OFFSET you can switch to a 64 bit platform.¹

vec EXPR,OFFSET,BITS
               Treats the string in EXPR as a bit vector made up of elements
               of width BITS, and returns the value of the element specified
               by OFFSET as an unsigned integer. ...

Or just split the EXPR into smaller chunks and do basic arithmetics for calculating OFFSET.

update

¹) doesn't help, according to BrowserUk

update

see older discussions and proposed workarounds

update

this workaround doesn't look efficient, since substr is an lvalue you can use constructs like

vec ( substr( $str, OFFSET_HIGH, 2**31), OFFSET_LOW, BITS ) = NEW
[download]

(well if substr allows OFFSETs that big ... can't test!)