Could you tell what sort of vegetation was covering the case: eg: dead flowers, or bits of bark, or bits of leaf? Can you determine what plant species it was on : Cootamundra Wattle?
Not all that practical, I'm afraid. The location is a round trip of around an hour's drive followed by a walk of around an hour then to find a needle in a haystack.
Can I encourage you to take a small plastic specimen bottle with you on outings, to take possible larvae into protective custody for rearing to get an ID?