I've had some commercial meads that I suspect have added glycerin due to a thick mouthfeel not explained by the amount of honey. I enjoyed it.
After doing some research on winemaking boards, I get the following consensus on glycerin additions:
Glycerin Addition for Body in Wine:
Suggested 1-2 TBSP food grade glycerin per gallon: Max 4 TBSP After 4 TBSP, it apparently starts to give a metallic flavor, so beware. (1 TBSP = ~15 ml, for those of us metrically inclined) This works out to 4-8 milliliters of glycerin per liter, 16 ml per liter is the max dose recommended. It is suggested that the wine is crystal clear with no chance of fermentation restarting as the yeast can metabolize glycerin. In mead, it would be best to clear and stabilize before adding glycerin. Most of these additions were suggested for big red wines, so mead may or may not need less to avoid flavor contributions.
No addition control, then a 2, 4, 8, 12, 16 ml glycerin per liter addition. Side by side taste test with a no addition control.
A single bottle of 3 year old, bone dry Perfect BOMM was used for this experiment. The honey sweetness has returned in this mead, but some additional mouthfeel would improve it. I poured 6 identical glasses with exactly 50 ml mead. Added the following additions in milliliter of glycerin per liter of mead:
0 - 0 ul - No addition control
2 - 100 ul
4 - 200 ul
8 - 400 ul
12 - 600 ul
16 - 800 ul - Max Recommended
Added the above amounts of glycerin with a pipet. Swirled each until the glycerin was completely in solution. Swirled the control as well to control for oxygen absorption.
0 - Classic OB BOMM. Very tart and bone dry.
2 - No difference detectable.
4 - Noticeable difference, but barely.
8 - This starts to get smoother. Adds a silkiness to the mead. Surprisingly, it also balanced the tartness a bit.
12 - A bit better than 8. More balanced with the tartness and silkier.
16 - The tartness is nearly neutralized here. Good or bad depending on your taste. Silkiness is about the same. It does feel "thicker".
Overall, the glycerin additions improve the overall mouthfeel and balance of the acidity of the mead. Dry mead is so thin that the max addition is needed for balance, but sweet meads would likely need less. I highly suggest a bench trial (like above) before adding to a full batch. The surprising result is that glycerin addition balances tartness. This fact is useful in the case of over-zealous acid blend additions!
I detected no metallic flavors even with the highest recommended addition. Sweetness was a slightly more pronounced, but very difficult to detect without side by side tasting. Perhaps going higher is possible without off flavors. I'll likely test this later.
Looks like we have another tool to add body to mead!