Subst TotalRisk = $shell(echo $Risk1 + $Risk2 + $Risk3 > /tmp/elog_out)

Subst <attribute> = $shell(gawk 'BEGIN{ print $Attrib1 + $Attrib2 + $Attrib3 }' ) 

No, you can't. The only way is to export a logbook to CSV, then import it in a spreadsheet program, then do the math there. 

 You can pass numerical attributes to an external command via "Execute new =...".  This external command can then do the math and pass the result _back_ to the logbook either as a new entry or as an edit with the command-line client "elog".

For example, given a new entry with (numerical) attributes A and B, if you have

Execute new = add_them.pl $A $B $message id

in your elog.cfg file, then $A and $B will be passed to the add_them.pl perl script.  Now have the perl script (or bc, or whatever you want to use) do the appropriate math and submit the result.  For example, your perl script might include

$C=$A+$B

$D=$A*$B

elog -a C=$C ... -e "$message id" "Sum as edit"

elog -a D=$D ... "Product as new entry"

Very good idea! I changed that in ELOG V2.7.5-2139. The fix will be contained in the next release.

There is one "feature" which causes this not to work. If an option is present several times (in your case "Show Attributes Edit"), the first one is taken which fulfills the current condition(s). So what happens is you select List=Personal, which triggers condition "1". Then you select "Dummy1", which triggers condition "11". Now elog goes top down, finds that List=Personal, so sets condition "1", then continues, finds {1} Show Attributes Edit = ... and takes this one. What I propose is to do the following:

Options List = Personal{1}, Work{2}
...
{3} Show Attributes Edit = List, Project, SubProject, Title, Priority, Context, Status, Due Date, Start Date, Alarm, Repeat
{3} Options SubProject = DummyChild1
{1} Show Attributes Edit = List, Project, Title, Priority, Context, Status, Due Date, Start Date, Alarm, Repeat
{1} Options Project = Dummy1{3}

Note that {3} Show Attributed Edit come before {1} Show Attributes Edit, so as soon as you select "Dummy1", the first "Show Attributes Edit" gets 
activated which contains "SubProject".