Getting references to internal entities from outside aggregate boundary

I have an entity 'Calibration' which is a variant entity under the
Aggregate 'MixDesign'. MixDesigns create calibrations and calibrations
are only valid as they represent a valuation based on mix design
properties. There is no such thing as a calibration without a mix design.

I have other objects outside the mix design aggregate boundary which
need to know which calibration is being used for their calculation,
but DDD dictates that no references to inner variants are held. 

I have considered moving Calibration to be its own aggregate but this
is awkward since a calibration really only exists as a valuation from
a mix design.

How have other people here dealt with this kind of pseudo-aggregate
reference problem? I really don't want to implement a service to deal
with associations in the domain for something as simple as object
references.
Should I reference the calibration by some kind of business key like
the Creation date (which is unique) or something?
thanks,
mike

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