Anistrophic filtering is available as a filter option in DirectX.
You can't really force trilinear at all. Mipmap transitions is the only place it is available as an option and those are special cases.
Here are the exact texture filter options available in DirectX.
D3DTEXF_NONE
D3DTEXF_POINT
D3DTEXF_LINEAR
D3DTEXF_ANISOTROPIC
D3DTEXF_FLATCUBIC
D3DTEXF_GAUSSIANCUBIC
The linear texture state gives the option to the video driver and hardware to determine if trilinear or bilinear is to be used.
Games that provide an option for trilinear filtering can only do so for mipmap transitions. Or they are using a proprietary hook into the video card driver. Or, they are using the terms to apply LINEAR versus POINT filtering, which are very different.
Yes, you can set a video card driver's filter options as that is where the decision is made as to what level of filtering will be available when the application asks for LINEAR filtering.