Calling his sources "Swiss uncodified customary law" doesn't seem to be very precise, it seems. Much of the law of that time was still based on cantonal law, as these only recently (in 1848, to be precise) became the modern Swiss Confederation. Previously, it was mostly a loose alliance of indpendent small states with in parts very different laws. The new constitution of 1848 clearly defined the topics where the confederation (or rather, its new parliament) was responsible for legislation. Laws about personality, property and inheritance, which are the main contents of the Zivilgesetzbuch (Swiss Civil Code) are such topics.
Thus there were codified sources, just not on a confederation level. And historic sources, based on Roman law, were also available, and Eugen Huber knew them, as he had researched that topic.
The German Wikipedia writes:
Zuvor hatte Huber eine vierbändige systematische Übersicht über die Geschichte und Gegenwart des Privatrechts der einzelnen Kantone erarbeitet (System und Geschichte des schweizerischen Privatrechtes, Basel 1886–1893). Den stärksten Einschlag im neuen ZGB fand das damalige, von Johann Caspar Bluntschli entworfene Privatrechtliche Gesetzbuch für den Kanton Zürich, das auch in der Ostschweiz rezipiert worden war, jedoch wurden auch Züge der auf dem Code Napoléon basierten Gesetze der Westschweiz und des Tessins sowie der auf dem österreichischen Allgemeinen Bürgerlichen Gesetzbuch gründenden Gesetze Berns, Luzerns, Solothurns und des Aargaus berücksichtigt.
Previously, Huber had written a systematic overview of the history and present of the private law of the individual Cantons. The strongest influence to the new Civil Code had the Civil Code of the Canton of Zürich, written by Johann Caspar Bluntschli. This had also be recited in eastern Switzerland. But also parts of the laws of Napoléon, in use in western Switzerland where used, as well as sources from the Tessin (southern part of Switzerland) and parts from the Civil Code of Bern, Luzern, Solothurn and Aargau, which based on the Austrian Comon Civil Code. [My translation]
That means the new law was merely an aggregation of existing, codified law, not of some random customary laws he overheard somewhere.
