Thanks for the update Hinnerk, in fact I was a little lazy to implement Vincenty's formulae (which would be the right thing) and went on with something like this, that turned out to be OK for the demo purposes:
select new ST_Point(:v_center, 1000004326).st_distance(new ST_Point(concat(concat(concat(concat('POINT ( ', in_long + 0.001), ' ') , in_lat), ' )'), 1000004326), 'meter') into v_dx from dummy;
Effectively, I find the value in meters for 0.001th of a degree at the point of interest and assume that it's going to be the same in the nearby area (10-20 km, for my scenario).
The use of multiple concats is nasty, though -- would there be a nicer way to create a geometric feature with SRID defined? Something like "new st_point(long, lat, SRID)" didn't work.
Lastly (since I got your ear here) -- is there a way to define a scalar variable of type st_geometry in SQLScript? I tried "my_shape type st_geometry", and it didn't work. Adding SRID to st_geometry didn't work either. What I'm trying to achieve here is to create some geometry that I'll use as a cookie cutter for further steps in the hdbprocedure. I could store that variable as a string, generate a geometry each time and combine it with other spatial functions, but I judge from experience that conversion from text to geometry (and back, too) is a rather expensive operation.