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On of the potential problems that I see is that these filters usually work by having a thin dielectric film that reflects the undesired IR wavelengths and these wavelengths are dependent on the film thickness. (In comparision, the UV cut is by absorbtion, not reflection.)
If the filter is tilted, the thickness of the film which is hit by the light changes and so there is a colour shift - you can easily see this by holding up a visible light filter interference filter to your eye and tilting it - the colour changes. It's also how most room-temperature solar H-alpha filter achieve their 'tune'. Probably, the IR filter at an angle would let through the IR and only the very long wavelength Ir would be reflected.
For optimum results, you would need to choose a filter with a shorter cutoff (for the normal orientation) to compensate. Alternativly, if there was enough long IR to operate the guider, you may need another correctly orientated IR filter to block the shorter IR.
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