I cobbled together a filter (for command line ATM, not plugin) for the sunbeam thread. It is kind of buggy and gross but maybe we could salvage something from it. 
The convolve becomes more expensive as dimensions increase, so I made the decision to resize, which contributes to the ugliness.
Edit: For those who aren’t familiar with G’MIC scripting or cannot bear to read the crude code, the filter has 5 parameters:
$1 → length of beam (not to scale; larger value means longer beam).
$2 → diagonal direction (choose: 0,1,2,3).
$3,$4,$5 → colour of beam.
beams_test: skip ${1==1}&&{2==0}&&{3==104}&&{4==220}&&{5==255}
r={w} r2dx 200,1
+l
+l repeat {$1*20}
+l.
if {$2==1} shift -1,1
elif {$2==2} shift -1,-1
elif {$2==3} shift 1,-1
else shift 1,1
fi
endl
+ c 0,1
done endl
100%,100%,100%,100%
if {$2==1||$2==3} gaussian. 10,1,-45
else gaussian. 10,1,45
fi
convolve.. . rm. + c 0,255
endl
l.
s c *... $3 *.. $4 *. $5 / 255 a c c 10%,100% n 0,255 * 3
endl
+f.. min(I)>0?I:I#1 k.
r2dx $r,1
The image again for your convenience:
