The export information isn't contained in a string it's a rich object. To access those properties you need to expand the SecurtiyRules property to view the full object.
[0:42]: Get-NaNfsExport /vol/boot_luns -Persistent | Select-Object -ExpandProperty SecurityRules
Anon :
Nosuid : True
ReadOnly :
ReadWrite : {all-hosts}
Root :
SecFlavor : {sys}
NosuidSpecified : True
Alternatively if all you want is to read the export file you can do that with the read-nafile cmdlet.
[0:43]: Read-NaFile /vol/root/etc/exports
#Auto-generated by setup Tue May 10 18:12:29 GMT 2011
/vol/root -sec=sys,rw,anon=0,nosuid
/vol/root/home -sec=sys,rw,nosuid
/vol/boot_luns -sec=sys,rw,nosuid
/vol/vol6 -sec=sys,rw,nosuid
/vol/WHS_Data -sec=sys,rw,nosuid
/vol/vmdata1 -sec=sys,rw,nosuid
/vol/vmdata -sec=sys,rw,nosuid
/vol/software -sec=sys,rw,nosuid
/vol/quorum -sec=sys,rw,nosuid
Hope that helps,
~Glenn