Westpac tends to consider corporate trustees from a commercial or business perspective. They don't look at it from a residential viewpoint. You still get resi rates, but they've got different assessment processes and criteria.
As a result, I've always been very reluctant to use Westpac for this type of deal and my understanding is that they won't go over 80% LVR, but I admit I've never bothered to try.
There are multiple lenders who are more competitive and will go to 90% for corporate trustees. What's the reason for wanting Westpac specifically?
About the only way I can think of to get around the problem is to change the trustee to individuals (yourself), do the deal, then change the trustee again. This process is quite complex in itself (changing the trust, changing names on titles, getting the bank to agree when you change back). I don't think I'd recommend this either.
Best advice I've got is to do the deal with a different lender.