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I can show that this is true for your "simple" case.

If g(x,y) ∈ C^∞(ℝ²) vanishes on x ≤ 0 then it decomposes as g(x,y) = a(x)G(x,y) where a(x) ∈ C^∞(ℝ) vanishes on x ≤ 0 and G(x,y) ∈ C^∞(ℝ²).

This can be shown by proving the statements below. They could possibly be standard results, but I've never seen them before. First, I'll refer to the following sets of functions.

• Let U be thet set of functions f(x) ∈ C^∞(ℝ) which vanish on x ≤ 0 and are positive on x > 0.
• Let V be the set of functions f: ℝ⁺→ ℝ such that x^-n f(x) → 0 as x → 0, for each positive integer n.

The statements I need to show the main result are as follows.

Lemma 1: For any f ∈ V, there is a g ∈ U such that f(x)/g(x) → 0 as x → 0.

Proof: Choose any smooth function r: ℝ⁺→ ℝ⁺ with r(0) = 1 and r(x) = 0 for x ≥ 1. For example, we can use r(x) = exp(1-1/(1-x)) for x < 1. Then, the idea is to choose a sequence of positive reals α_k → 0 satisfying ∑_k α_k < ∞, and set

$$g(x) = x^{\theta(x)},\ \ \ \theta(x)=\sum_{k=1}^\infty r(x/\alpha_k)$$

for x > 0 and g(x) = 0 for x ≤ 0. Only finitely many terms in the summation will be nonzero outside any neighborhood of 0, so it is a well defined expression, and smooth on x > 0. Clearly, θ(x) → ∞ and, therefore, x^-n g(x) → 0 as x → 0. It needs to be shown that all the derivatives of g vanish at 0 so that g ∈ U. As r and all its derivatives are bounded with compact support, r⁽ⁿ⁾(x) ≤ K_nx^-n-1 for some constants K_n. The n^th derivative of θ is

$$\theta^{(n)}(x)=\sum_k\alpha_k^{-n}r^{(n)}(x/\alpha_k)\le K_nx^{-n-1}\sum_k\alpha_k$$

which has polynomially bounded growth in 1/x. The derivatives of log(g) satisfy

$$\frac{d^n}{dx^n}\log(g(x))=\frac{d^n}{dx^n}\left(\log(x)\theta(x)\right)$$

which also has polynomially bounded growth in 1/x. However, the derivative on the left hand side is g⁽ⁿ⁾(x)/g(x) plus a polynomial in g⁽ⁱ⁾(x)/g(x) for i < n. So, induction gives that g⁽ⁿ⁾(x)/g(x) has polynomially bounded growth in 1/x and, multiplying by g(x), g⁽ⁿ⁾(x) → 0 as x → 0.

By definition of f ∈ V, there is a decreasing sequence of positive reals ε_k such that f(x) ≤ xⁿ for x ≤ ε_n. We just need to make sure that α_k ≤ ε_n+1 for k ≥ n to ensure that g(x) ≥ x^n-1 for ε_n+1 ≤ x ≤ min(ε_n,1). Then f(x)/g(x) goes to zero at rate x as x → 0. █

Lemma 2: For any sequence f₁,f₂,... ∈ V there is a g ∈ U such that f_k(x)/g(x) → 0 as x → 0 for all k.

Proof: The idea is to apply Lemma 1 to f(x) = Σ_k λ_k|f_k(x)| for positive reals λ_k. This works as long as f ∈ V, which is the case if Σ_k λ_ksup_x≤kmin(x,1)^-k|f_k(x)| is finite, and this condition is easy to ensure. █

Lemma 3: For any sequence f₁,f₂,... ∈ V there is a g ∈ U such that f_k(x)/g(x)ⁿ → 0 as x → 0 for all positive integers k,n.

Proof: Apply Lemma 2 to the doubly indexed sequence f_k,n = |f_k|^1/n.

The result follows from applying lemma 3 to the triply indexed sequence f_i,j,k(x) = max{|(d^i+j/dxⁱdy^j)g(x,y)|: |y| ≤ k} ∈ V. Then, there is an a ∈ U such that f_ijk(x)/a(x)ⁿ → 0 as x → 0. Set G(x,y) = f(x,y)/a(x) for x > 0 and G(x,y) = 0 for x ≤ 0. On any bounded region for x > 0, the derivatives of G(x,y) to any order are bounded by a sum of terms, each of which is a product of f_ijk(x,y)/a(x)ⁿ with derivatives of a(x), so this vanishes as x → 0. Therefore, G ∈ C^∞(ℝ²). █

In fact, using a similar method, the simple case can be generalized to arbitrary submersions.

Let p: M →N be a submersion. If h ∈ C^∞(N) and g ∈ C^∞(M) satisfy hg = 0 then, g = aG for some G ∈ C^∞(M) and a ∈ C^∞(N) satisfying ha = 0.

Very Rough Sketch: If S ⊂ N is the open set {h≠0} then g and all its derivatives vanish on p^-1(S). The idea is to choose a smooth parameter u:N-S →ℝ⁺ which vanishes linearly with the distance to S. This can be done locally and then extended to the whole of N (I'm assuming manifolds satisfy the second countability property). As all the derivatives of g vanish on p^-1(S), u^-ng tends to zero at the boundary of S. This uses the fact that p is a submersion, so that u also goes to zero linearly with the distance from p^-1(S) in M.

Then, following a similar argument as above, a can be expressed a function of u so that g/a and all its derivatives tend to zero at the boundary of S. Finally, G=0 on the closure of p^-1(S) and G=g/a elsewhere. █

I suppose the next question is: does proving the special case above of a single g and h reduce the proof of flatness to algebraic manipulation?