You could do it by placing a duct fitting, a union. If necessary, you could customize the family to create some lips or flanges. The default union family would look like in this image, below, which might be good enough. You can place the unions at the intersection of reference planes and the center line of a long duct. Then, you can schedule all the individual segments of 1.5 m. long and tag them all as "501", customizing the duct tag to read the value from another parameter that is not the size.

Alfredo Medina,

Thank you for the reply! It's the first time I've come accross the "union" family - I guess if one doesn't use it, one doesn't see it! Thank you for the info and I'll give it a try!

The quick way to place unions is to set the type of union you want in the duct system routing preferences and then use Modify -> Split on the ducts.

However, placing all those unions is still going to be a tedious job. You could try making just the flange as a Duct Accessory family. Make it an Attaches To type of accessory and then array the flanges inside the family so they appear every 1.5m between the two end reference planes.
Then you could place the family on a long duct, stretch the two end reference planes to the ends of the duct (with the grips or Align) and all the flanges in between would appear.

If the intention is to mimic a fabrication process or at least model to support a more realistic fabrication process then I'd be inclined to add the "flange" geometry to a custom union family so placing a union creates the break between sections and provides the more realistic appearance of a flanged connection.

Steve_Stafford, the intention is both. Our company manufactures some ducting and installs HVAC systems. The intention is to indicate to the folks on site which number (tag) of duct goes where as well as quantity/manufacture purposes.
Our installations can be massive and rats-nest-like so the duct segments are numbered (as per the plan) as they're manufactured and when they get to site the guys there know where the piece goes as they have the same plan with the duct segments and numbering on. There should be no errors or misunderstandings (and there usually aren't) to where a duct (or duct run with the same tag numbers) should go. The duct with the "odd" size can also be shown on plan to best suit connection and areas that may need a little play in reality.

Not probably the most traditional use (engineering firm) for Revit but I believe it'll have its merrits in the manufacture and installation industry too. My boss is mostly convinced - I just need to push a little and show him some more of it's merrits.