Both Poland and Czechoslovakia had large populations of ethnic Germans, so it seems reasonable that a few of them may have found their way into the armed forces in some capacity or another. However, I also know that the relationship between the ethnic German populations of both countries and the respective states was tense at best, which may have led to policies against this.
Nevertheless, if we do know of any who fit the description, what would have happened to them? Unceremoniously shuffled out as diplomatic relations deteriorated? Shot as blood traitors by Hitler's forces? Integrated into the Wehrmacht as it pushed through?
In Second Polish Republic in 1931 ethnic minorities were 30% of citizens of Poland (Germans were ~2%). As all citizens, they were obligated to serve in the Polish Army but they were subject to some restrictions. They couldn't be officers. They could be NCOs but there was a limit - only 5% of NCOs and 30% of privates in a unit could be from ethnic minorities. Ethnic minorities also couldn't serve in some military branches such as armoured force, navy, air force or signal corps. Ethnic minorities were sent to garrisons far away from their home, for example Ukrainians and Belarussians were sent to Western Poland and Germans were sent to Eastern Poland. Although in September 1939 almost all Polish units were sent to the West and only part of the Border Protection Corps (KOP) remained on the border with the Soviet Union.
Polish soldiers of German ethnicity which were captured by the Soviets were sent to Germany. I haven't heard about any reprisal for ethnic Germans who served in the Polish Army.
Germans in Poland and Poles who were viewed by the Nazis as Germans were obligated to sign Volksliste. They were divided to four categories:
People of German descent who were loyal to Germany before 1939.
People of German descent who were passive and neutral towards Poland and Germany.
People of German descent who were partly polonised (for example they married Poles). Most of Silesians and Kashubians were also in this category.
People of German descent who were fully polonised.
All able man Volksdeutsche had to serve in Wehrmacht. Many of them, especially those from two last categories, deserted and joined Polish Armed Forces in the West or in the East.