We are considering a money market product where we have a 10% "resereve" (only paying interest on 90% of the funds) which will be fully disclosed. Is it safe to say that the APY is the same whether it is calculated on 100% or 90% of the balance?

This was common practice years ago and outlawed by Regulation DD:

Section 1030.7--Payment of Interest
(a)(1) Permissible methods.

1. Prohibited calculation methods. Calculation methods that do not comply with the requirement to pay interest on the full amount of principal in the account each day include:

iii. Paying interest on a percentage of the balance, excluding the amount set aside for reserve requirements (the “investable balance” method).

Nice try though.

So if this is a tiered account, does this mean that we cannot pay 0% on the first tier as long as it it disclosed properly?

That is a horse, as they say, of a different color. If, for example, you have a 0% tier from $0 to $1,000 and pay 0.10% APY on balances above $1,000, you aren't failing to pay interest on the "reserve" portion of the balance. And, as Appendix A to Regulation DD points out, your interest could, using tiering method B, only apply to the portions of the balance above $1,000, so that the actual APY for the upper tier would be in a range starting at 0.00% and approaching 0.10% as balances increased.

It may be a subtle difference, but it is a difference.