To the point of including carbonation of cementitious materials, but still ignoring biogenic carbon. I like to differentiate between the two as “carbon sequestration” (carbonation) and “carbon storage” (biogenic). Carbon sequestration is long-term carbon storage (~1000+ years), whereas carbon storage is less permanent (i.e., timber). The carbon sequestered by concretes and mortars will stay in the form of calcium carbonate until it is heated up again to high temperatures (either by humans, or by a geological process) which is unlikely to happen in the near future. Carbon storage on the other-hand describes the temporary storage aspect of biogenic carbon, since trees/grasses decay if left to the elements, and the carbon (or methane) is released back. I recognize that I’m omitting a lot of nuance that comes with forests and natural carbon cycles, but this is my general take on the topic. So within this paradigm, biogenic carbon uptake, and cementitious carbon uptake should be considered separate, yet in their accounting, they can be treated the similarly (idealy with dynamic LCA). For example, I like this study which accounts for both biogenic carbon and cementitious uptake with dynamic LCA (https://doi.org/10.1016/j.buildenv.2017.12.006).
There is not good consensus on how to treat the dynamic aspects of LCIs and LCAs that you raise Will. I find that Charles Breton has done a great job summarizing the discussion around accounting for the temporal aspect in this open-source review: https://www.mdpi.com/2071-1050/10/6/2020 (see Table 5 for a good summary of all approaches people have taken).
While discounting can be applied (see references in Breton et al.), a GWP100 metric (the common metric used in LCIs/LCAs), already accounts for the cumulative impacts of a pulse of a greenhouse gas emissions at t_0 for 100 years, including both direct and indirect impacts. So, the future effects of a GHG emitted today is included in the metric (albeit with many caveats that I’ll leave to the climate scientists to discuss - this is how the IPCC AR5 report handles the issue though - see Ch. 8). Is the carbon discounting that you are applying accounting for this effect already included in GWP100? Or is it something else? Is the 1.4% value you reference for the economic cost of climate change mitigation, or for greenhouse gas emissions? If you could point me to resources on this, I’d appreciate it as I don’t have much familiarity of discounting in the context of GHG accounting.
Now, how this now applies to quantifying the carbon uptake of concretes and mortars in a static LCA. In our simple screening LCA, we have treated the GWP of future carbon the same as the GWP of present day carbon. In reality, the two values do not have the same time horizons and need to be differentiated. With timber, others have treated this difference with a GWP_bio metric (for use in a static LCA). For carbonation GWP, a similar metric, say, GWP_carbonation could be applied. This is a metric that I plan to develop in the next year or so that builds upon the carbonation work Wil linked previously (more to come 
).