You can do the following

1) Create RenderTarget in D3DPOOL_DEFAULT
2) Render LightMap
3) Create temporary texture in D3DPOOL_SYSTEMMEM
4) Use GetRenderTargetData to copy data from D3DPOOL_DEFAULT to D3DPOOL_SYSTEMMEM
5) Destroy RenderTarget
6) Create ordinary texture in D3DPOOL_MANAGED
7) Use UpdateTexture to copy data from D3DPOOL_SYSTEMMEM to D3DPOOL_MANAGED
Destroy temporary texture
9) Use managed texture, and forget about device reset!

The first thing: I haven't tried all of this, but it should work
The only drawback of this approach is that it's slow. But since it's slow once, if that isn't problem for you...