I'm not sure this will help too much, but I've found a couple of Italian sites that also offer English-language versions. They refer to "tassellato" as "with inserts", "with loose parts", "with loose pieces". I'm not sure exactly what they mean by this, though. Here are a few extracts:

• Vasi sifonici a brida riportata
• Vasi a cacciata con brida riportata
• Vasi a brida aperta
• Vasi a brida tassellata (a cacciata o sifonici)
• Attached rim siphon WCs
• Flushing WCS with detached rim
• Open rim WCs
• Box rim WCs with inserts (with flushing or siphon)
http://www.garoll.it/Italiano/Home-Italiano/Prodotti/pdf/lem...

L’impianto HPC-401 consente il colaggio in alta pressione di:
􀂉 vasi a brida chiusa con tasselli
􀂉 Vasi a brida aperta
􀂉 bidet
utilizzando stampi in resina microporosa in 4 o 5 parti (a cui si aggiungono gli eventuali tasselli)
The HPC-401 pressure casting system enables high pressure casting of:
􀂉 Box rim WCs with loose parts
􀂉 Open rim WCs
􀂉 bidets
using microporous resin 4 or 5-part moulds (to which loose parts, if any are added))
L’unica ed eventuale operazione manuale rimane, nel caso di vasi a brida tassellata, l’estrazione ed il riposizionamento dei tasselli sul pistone di brida.
In the case of WCs with rims equipped with loose parts, the only manual operation is removal and re-positioning of loose parts on the rim core.
http://www.garoll.it/Italiano/Home-Italiano/Prodotti/pdf/hpc...

Stampi in LOPREM per vasi a brida aperta e a brida chiusa tassellata (box rim produced with loose pieces) sono stati a lungo testati prima nei laboratori della SACMI, poi a livello industriale presso alcuni clienti interessati all’applicazione.
http://www.sacmi.com/FilePdf/1/1/0.632113132493437500.pdf

Of course, this doesn''t help at all with your other term "castellato".