Can the Amyloglucosidase Reagent (R-AMGR3) be used to measure Amyloglucosidase (E-AMGDF) levels in malted barley and what is a suitable extraction procedure to use for ground malt?

The amyloglucosidase (AMG) present in ground malt is most probably of fungal origin, i.e. A. niger.  The flour should be extracted with 100 mM sodium acetate buffer, pH 4.5 at 1 gram flour per 10 mL of buffer.  You should note that the PNP-beta-maltoside substrate is also susceptible to hydrolysis by alpha-glucosidase, so it would be difficult to know which of these enzymes you are measuring.

Product Page (R-AMGR3)