The first correspondent, Jan Kalendovský, confirmed that both Smejkal and Uhlmann qualified from 1976 Arandjelovac for the Interzonal phase after the drawing of lots. He also forwarded a column by GM Hort from a May 1976 issue of Rudé právo which discussed the outcome. I added the clipping to my page on the cycle, Zonals 1975-1978 (C10).
Rudé právo (Czech for The Red Right or The Red Law) was the official newspaper of the Communist Party of Czechoslovakia. Rudé právo [Wikipedia]
The second correspondent, Jan Fiala, commented on the circumstances surrounding the boycott of 1975 Barcelona.
The theory about Pachman is possible. Two chessplayers in the 1970s couldn't play for two years because they played in one tournament with Pachman -- Ján Báňas and Ivan Novák, Stockholm 1974 -- but I didn't read about it anywhere [else]. Moreover, Smejkal in his memories in one interview wrote about it and I think if it was because of Pachman, he would say so.
He also forwarded a translated excerpt from 'Jan Smejkal - Bretislav Modr: Smejkalova padesatka, Sachinfo 4/1996, page 136'.
In time when I was on top and I saw the whole world pink, I should play in next cycle zonal tournament in Spain (in year 1975) and I did not. The day before take-off a telegrame came that tour is not held. And a declaration was published without our knowledge together with Yugoslavians, Poles and Hungarians in which we protest against Franco's regime.
Zonal tournament was held and Ludek Pachman, who lived in Germany yet, promoted [i.e. 'qualified'] amongst others. [...] Finally I promoted to interzonal, together with interest of Russians and Americans but also of organizers of interzonal tournaments and next promoting places were enforced for those who could not start in Barcelona. I promoted but without illusions.
Jan Kalendovský is a published author; see Complete Games of Alekhine on newinchess.com. Jan Fiala maintains a web page on Czech chess history at sachyceskybrod.cz/historie.htm. Thanks to both for the information.
Later: I added another clipping, received from the same correspondents, to the 1975-1978 (C10) page. It's under the heading 'Barcelona 1975', captioned 'Rudé Právo 1975-10-03'. A translation was also provided:-
As a Protest Against Reprisals Yugoslavian chess master D. Velimirovic anounced to FIDE secretariat that he will not take part in zonal tournament in Barcelona as a protest against fascist regime in Spain. At the same time he asks international federation to replace the tournament to other European country.
The correspondents mentioned that there were other published protests.