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The matter of historical characters is still in flux. Many have already been added to the Unicode Standard but there will still be some that are lacking from the Standard and hence from available fonts. Alternative forms of characters may be composed using Adobe’s SING (Smart INdependent Glyphlets) Gaiji plugin for Japanese InDesign, but this has not been tested by Brill or its typesetting partners. See the information from Adobe: SING Gaiji. This looks like the best technology, which also preserves Unicode encoding and therefore searchability. Although the technology seems to be geared towards the Japanese-localized version of InDesign, it can doubtless be applied in Chinese text as well. The glyphlets can be ported to non-Japanese versions of InDesign.

Please also consult CJKV fonts: How to choose between them.